e.g. the [link|http://www.usr.com/products/home/home-product.asp?sku=USR5686E|external V.92 modem]. Around $95.
After being bought by 3Com, USR was sold off. I don't have any experience with the post-sale products (I've got an external V.90 Voice/Faxmodem), but I've not heard anything bad about USR modems in a long time.
You might want to check eBay. Zoom and Supra are other brands to consider, but just about any external modem should work equivalently.
Cheers,
Scott.