Nitrazine is totally useless for determining rupture. It's basically a pH swab, so urine or alkaline vag fluid (which it often is in PG due to changes in flora) will also change the color.
I hope you don't mean you're checking to see right now! You're only a few weeks along if my memory serves me correctly! Anyway i have no idea where to find them. Find a midwife, ask for some. She'll probably hook you up.
Hmmmm....I'm not sure everyone agrees about the nitrizine. I do know you can get a hand lens, collect the fluid as it comes out, and look in the lens for ferning....That would be a better test than the nitrizine paper.
Try googling midwifery supplies....
Or go to www.birthwithlove.com This is where I got my birth kit for my first baby. Lots of good stuff on that site. NAYY.
I've never found nitrazine to be useful for diagnosing rupture, since it's so nonspecific, but ferning is a very specific diagnostic tool that's very easy to recognize.
Follow Mothering