DS #1 is getting to the age where he LOVES doing science experiments, would do one every day if I'd let him! So far I've purchased the items individually, but I can forsee purchasing kits in the future. So far I've just checked books out from the library and pulled together what we need (right now we are using a great book called Bathtub Science).
I've seen Christian Book Distributors sell science related items fairly inexpensively. You might also want to check out Discount School Supply, they sell science related things as well. Lately the last few garage sales I've went to have had science kits, microscopes, etc so that might be an option. Target used to sell National Geographic kits in the toy section that are basically science kits, so that might be something to try as well.