This should work:
http://www.rei.com/product/603034

However, are you sure that the seams are the only place water is coming in? If it's leaking elsewhere, you may be better off getting a lightweight nylon tarp to pitch over the top of it, as a rainfly (or second rainfly.)

I once had a cheapo single-wall pup tent from WalTar-K. It leaked in the first rain, too; I think the seams were the only place water didn't come in. An inexpensive poly tarp (nylon tarps were luxury gear, in those days) pitched over it solved the problem. Then I realized it was better ventilated and easier to leave the tent home and just use the tarp.