Does the service manual give a part number? If yes you can try the Mitsubsihi Parts Deptartment. 1-800-553-7278. If it does not give a part number, most likely it is only a part of a total assembly which most likely is more expensive.
That part, the polarization splitter converter, is standard in any 3-LCD design tv, any manufacturer. I believe they are all the same dimensions as well, probably all made by the same subcontractor. Just make sure you get the tiny prisms pointed in the right direction when you put it back in.
Acceptable donors would be any 3-LCD Mits, any Sony 3-LCD (not SXRD), any Hitachi, Zenith, or Panasonic 3-LCD. Not JVC.
Some of these, like the Sony 42/50/55/60WE or WF/610 or 655, are pretty easy to find on craigslist for cheap.
I don't often see these burned up, perhaps it's a problem with Mits cooling.