A wifi repeater is what you're looking for. It, essentially, takes a weak signal, amplifies it, and rebroadcasts it. The caveat is, though, you sacrifice some speed for it. If that's okay with what you need, then you can find some at any electronics store.
Edit: Another option would be to add a high-gain antenna to the router. I don't know much about these, though. Bought one at a garage sale for $10, but never used it.
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