Typically, maternal antibodies last
for about 2 weeks after the
kittens stop nursing, so when she weans them, make sure you get them their kitten shots within 2 weeks of weaning.
There is a product called KMR (Kitten Milk Replacer) that they sell in pet stores, and there's also a product called Just Born, which is a powder that you mix up to make a milk replacer. Both are available in grocery stores and pet stores.