When Should She Be Spayed?
I wasn't able to find any definitive information about when a nursing mother should be spayed, but spaying nursing mothers seems common in TNR (trap neuter return) groups (it's better to spay a cat that you have in a trap now, then try to trap her again later).
Ally Cat Allies (a leading TNR group) states (based, presumably, on their 25 years of experience in TNR advocacy):
Nursing mother cats continue to produce milk after being spayed, and can continue to nurse their kittens.
Best Spaying Technique for Nursing Mother
If you want to get her spayed while she is nursing, you can call the local veterinarians and ask if they are experienced in flank spaying.
In the US, cats are normally spayed with an incision along their midline (the belly), but in UK (at least in 2006) the incision is more commonly located along the flank (along the side).
The benefit of a flank spay for a nursing mother is that the incision is less likely to be affected by any leaking milk, and you will be able to monitor the incision without bothering the kittens.