Where can I find reusable white playing cards for prototyping?
While it doesn't directly answer your question of where to get reusable playing cards, I have helped friends play-test card games before, and we have always just grabbed a handful of junk Magic common cards (because who doesn't have thousands of those lying around?), put them all into sleeves, and then slipped pieces of paper between the face of the card and the sleeve.  One of my favorites is taking a stack of note cards, cutting them in half, and using each half for a card face.

Now you can just change out the slips of paper when you want to update a card, and you can just recycle the old slips of paper afterwards.  When we've been short of paper, we've even cut up grocery bags, or written on the backs of receipts or used envelopes, its all the same.

Once a card is nice and stable, you can print off a nicer version and slip it into a sleeve and card if that's what you want.