Howdy - I couldn't find cellcom's roms. This thread (For a different model phone) might have something useful.
http://forum.ppcgeeks.com/showthread.php?t=42005
From what I understand, 99% of the rom you have will be the generic stock TP2 rom, and there are just a few tiny custom carrier settings. The thread had some advice on how to find and back up those carrier settings.
Here's the generic rom from HTC's site.
http://www.htc.com/europe/SupportDow...at=2&dl_id=913
You other option is to use one of the backup programs. SPB backup, sprite, resco, and SKtools all make backup software. Sprite also makes 'clone' software that actually images the whole thing bit for bit. This is ideal but I think you'll have to pay for such software. There may be others who make similar ghosting software.