I read this question that claims that alfalfa hay is a "treat" and should not make up a large part of rabbit's diet. Is this true?
Any grass hay will work. Timothy hay is difficult to come by in large qualities where I live. However mixes of Orchard, Brome, and other grass hays are more common. Timothy is a great hay because it contains a great balance of nutrition without calcium and it is low in fat and calories. Alfalfa is high in calcium and calories in comparison. Some other grass hays to avoid for regular feeding is Clover. Clover is high in calories and is not good for a domestic rabbit.
Here is a comparison of common grass hays from the San Diego House Rabbit Society:
Expanding a bit on Critters great answer.
Alfalfa has about 3 times as much calcium as timothy hay ref with clover about the same or higher in calcium. Both alfalfa and timothy have about the same amount of energy and nutrients, with alfalfa being slightly higher. Alfalfa has about twice as much protein as timothy. ref(pdf)
Rabbits are dependent on having a relatively high fiber, low food value diet ref. If the diet is too high in protein they will tend to not eat their cecotropes which also meet other health requirements. So for a normal healthy domestic rabbit, who is not breeding and living in your home for the next 8 - 12 years, you should consider alfalfa and alfalfa products as treats.
Young rabbits who are less the 8 weeks old, should get alfalfa hay and/or pellets ref to help with the development of strong bones. At 8 weeks they should be transitioned to timothy hay and pellets. Breeding mothers also require the higher calcium content of alfalfa.
Rabbits are one of the few mammals that excrete excess calcium through the urine. ref Diets high in excess calcium, like a pet rabbit getting lot of alfalfa, are more likely to suffer urine sludge, stones in the bladder and/or kidneys.
Also keep in mind that if you are feeding pellets, the pellets should be timothy hay based. There are products with alfalfa base, the same considerations that apply of alfalfa hay apply to alfalfa based pellets