This answer is based on the situation in the Netherlands. I'm not up to speed with the exact Belgian laws, but I do know that there is quite an active trade in reptiles between the two countries.
I'm adding the "in Europe" because I don't know if there's any anacondas being bred outside of South-America
They are sold every now and then, which seems to imply that they are also bred here. A pet store close by my parents' home had a Green Anaconda (Eunectes murinus) for sale for 125 euros.
For keeping snakes in Belgium, according to this website, species that can be kept as pets depend on their CITES indication. CITES is an international organisation that determines the level of endangerment of a species. There are several levels, that dictate whether they can be kept by individuals, with or without permits, or only by zoos and other official organisations. All of the Eunectes species are level II on the CITES list. This means that their trade is regulated, but this is a very broad definition. In practice this means that whenever you buy a snake of these species, a form has to be filled, indicating the buyer's and seller's identity and the origin and age of the animal. The important distinction here is to make sure that the specimens are captive-bred, instead of wild-caught.
So what are important things to know about keeping Anacondas?
Species
There are four species of Anaconda:
Eunectes murinus, the "classic" green Anaconda. These are the massive giants that are tied with the Python reticulatus for the title of biggest snake in the world.
Eunectes notaeus, the yellow Anaconda. This is the green's little cousin. Whereas the greens can peak at 9m (but generally stay around 5m) and 100kg (but generally 30-70kg), these stay a bit smaller (3-4.5m and 25-35kg).
Eunectes deschauenseei and Eunectes beniensis, two relatively rare species. These are too rare to expect them to be kept as pets, especially in Europe.
Big snakes
Ok, so Anacondas are big snakes. While the yellow anaconda is smaller than the green anaconda, they are both Big Snakes™. At adult size, these are big enough to pose a danger to humans. As such it is highly recommended to never handle these snakes alone. It is not worth the risk. This is a similar advice to handling Python reticulatus, P. sebae, P. bivittatus and large subspecies of Boa constrictor. Generally though, due to their sluggish form and lifestyle (more on that later), they should be relatively laid-back. This is evidenced by comparing footage of wild encounters with Anacondas with their gigantic python counterparts P. reticulatus. However, they remain wild animals that could overpower a human if they put their mind to it.
Lifestyle
Anacondas live in the Amazon rainforest. Here they spend most of their time in the water, hunting on large mammals that live in and around the water. This is how they can become as heavy as they are. If you see an anaconda on the dry, they are really sluggish, because they have to drag all that weight along with them. In the water though, they have none of these issues. So anacondas like water.
When keeping them, this can be easily accommodated by placing a large tub of water in their enclosure. Make sure it can't be knocked over, because else it will be knocked over.
Recommendation
I already mentioned that anacondas are big. I wouldn't really recommend keeping either as first-time snakes. It's better to start out a bit smaller, mostly so you can learn how to read body language. With a smaller species, like Python regius or Boa constrictor constrictor, misreading it might mean a bit and some shed blood, but it won't put you in any real danger.
Body language
Learning to distinguish inquisitiveness from a defensive posture from looking for food is very important. Normally, snakes are just looking around, checking out things around them. This is perfectly fine. If they feel threatened, though, it is important that you recognise this. If not, if you push them too much, you might end up being bitten out of self-defence. This is generally not the end of the world, but it might put you in the hospital from the puncture wounds from the bite. If they are looking for food, though, this is definitely the most dangerous. You don't want them to think you are food. Most of the time, this is by mistake, either because you smell like food (if you recently handled food items for example), or something else might have triggered their food response. But that is mostly irrelevant. You don't want them to try. In contrast to a defensive bite, a food bite will mean that they will hold on and that they will try to constrict.
At that point their sluggishness is completely gone, and they will work entirely against you. 50kg+ of pure muscle is not something you can overpower by yourself. This is also why it is so important to have someone else with you at all times.
This may sound grave and grim, but it really is not. Snakes are generally pretty easy to read, once you know what to look for. However, starting with smaller species definitely is the more forgiving route to take there.
Back to recommendations
So when/if you have some experience keeping snakes, the next step is to get experience with Big Snakes™. From here, the yellow anaconda is relatively more easy to keep, due to its smaller size, but it mostly depends on what you are comfortable with. It's really no good keeping snakes you're not comfortable handling. This only ends in a downwards spiral with them not being used to being handled because you don't do it, making them react weirdly to it, making you not handle them as much, etc.
Some random notes
I've heard it told that anacondas are pretty messy and stinky snakes. They are used to living in the swamp, which also means that they're pretty much being used to their mess just being swept away (snakes are actually surprisingly tidy, if they pooped on their favourite spot, they will move away, so they don't have to lie in their poop. Of course this won't stop them from pooping in their favourite spot in the first place).
I have also heard that they're not the easiest snakes to keep in terms of requirements and hygiene.
