The question title says it all : Where can I find reusable white playing cards for prototyping?
I find services online to generate cards but not reusable white cards (that you can write on and erase later).
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.
That sounds very challenging, given that your erasable cards would need to NOT erase themselves during shuffling.
I'm designing a card-heavy game myself, and at the moment, my plan for prototyping is to print the cards on light cardstock (I have a laser printer) and then cut them out manually. I don't expect them to last particularly long, but for the prototyping phase, I don't need them to. The downside is it's a bit labor intensive (though I figure a small guillotine-style paper cutter will ameliorate that), but the cost in materials will be negligible. Would something like that be an option for you?
Plastic Projector Sheets Cut to sizes you need. You would only be able to use dry markers, or maybe ink pen, but whichever surface you decide to use pen marks would probably be left behind.
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I use this all the time for flowcharting. (You can put a white sheet of paper behind each sheet.btw)
One technique that works ok is to get an old pack or two of playing cards and a roll of sticky labels, and just stick labels over the faces. You only get one use, but the labels are quite easy to peel off and replace.