A good start would be in the Touch Pro section. And because Bell is a cdma company, you would want a cdma rom. do not try to flash a diamond rom, you will brick your phone. same thing if you use a gsm rom of any kind.
http://forum.ppcgeeks.com/forumdisplay.php?f=68
secondly, forum rules state that this kind of post does not belong in the upgrades section!!! only roms or developed upgrades can be posted here -- save your questions for the regular forums (in this case, the HTC Diamond base forum or the HTC Touch Pro base forum..)