T-mobile Equipment Protection?
This is a long shot, but check (most likely you know this or not) if you have T-mobile Equipment Protection as an option.
I checked
http://support.t-mobile.com/knowbase/root/public/tm22023.htm?
Its a $110 deductible (assuming T-Mobile US) for the MDA, so if you have damage that affects use of the phone, it may be worth paying the deductible and obtaining a replacement.
As for buying case plastics for a do it yourself replacement, I have zero clue here.
(Edited for spelling mistakes.)