Could you, or a friendly person with a suitable printer, simply use one of the many free alphabet flashcard designs? You can't get much more indifferent than blank backs.
I had a quick search and there are a lot out there, but many have pictures, or look a bit too childish. This one had a good selection and note the one that is in "portrait" orientation, which is described as a children's book font.
http://havefunteaching.com/flash-cards/ ... ash-cards/You'd also have as many spares as you could print out. It might not be ideal having to get both lower and upper case letters, but you could have two sets, I suppose. Sizing might be an issue too, but again that can be a problem with using a real set of kids' cards.