Best set of blank cards
I was poking around the other day looking for the same answer. Plaincards.com sells software to design your own cards and blank cards to print your designs on. Seems pretty cool, but it looks like you're limited to standard playing card sizes (i.e. no tiny Arkham cards). Their complete product line appears here. I'm not sure what size cards Bohnanza uses.

I'm in the midst of designing my own game but I need different sized cards (1.5" by 2.5") so their apparent lack of that size is kind of a deal breaker.

DeLano Service is another site that could get cards made. Sounds to be a bit more fancy that what you're looking for, perhaps more like a prototype company that an supplier of blank cards, but perhaps they'd sell blank cards if asked.

(Note: I have no affiliation with either site.)