Do you want big ones or little ones? If you want little, you can go to Target or BRU and get the little sets of them. Normally I think there's 10 in a set. If you want large ones, maybe try Amazon and just order more?
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Re: Where can I buy...
Do you have any kind of a dollar store? I'd check there first then hit up the Target dollar spot. They almost always have board books there.
Try Oriental Trading Co. online. They have all sorts of stuff you can use as favors and they're bulk and cheap.