I'd wait to get a birthing supplies until you are certain she is going to be a mommy. Remember, many cats give birth on their own with no help. Breeders do this frequently and often our cats are expensive purebreds so we are a lot more cautious of our queens. You can also ask your vet what he suggests closer to the due date.
Given you only plan to do this once, you may not want a whole kit. But some of the things breeders have in their kits are:
for the birthing box:
-plastic waterproof liner on the bottom, pee pad in the middle, towels on top
vet number & after hour number close by phone
heating pad/hot water bottle to keep the kittens warm after they are born and while mom is still working (these go in a small box, pad on low, towel on top)
small scale to weigh the babies, pen and paper to record (breeders often have newborn id collars)
small sterilized scissors, alcohol swaps, and dental floss (in case you have to help with umbilical cords)
nasal aspirator (in case you have to help clear mucus so the baby can breath)
kitten formula (mom may like some while she is birthing...babies may need some if mom doesn't start or isn't able to start nursing) also may want to have a baby kitten bottle available (again...these are things that breeders have, you may not want/need them)
hand sanitizer (for you)
a good book to read if you stay with her...coffee if you need to stay up all night
I haven't used crystals...but I guess if they don't clump. Just watch the babies to make sure they don't eat them! There is kitten attract litter (but I don't really like it)...many of the breeders I know use non-clumping walnut shell, corn, wheat.