If you really can't afford to get it on ebay or from sprint, your next solution is buying a plastic case. it still won't cover and protect as much as the original battery cover.
Having said that you sure you can't afford it? I see at least one on ebay for $10.
http://cgi.ebay.com/OEM-HTC-Touch-Pr...item27b22aa0ec
Looks like that one will fit Sprint TP2. I am sure there are definitely ones that will fit ATT TP2 if that one doesnt for comparable price,,,
You can spend a little extra ($15) and get an extended battery with cover also... Definitely not too bad if you already spend close to $200 on your phone