This where you can get a replacement:
TriMark Service & Replacement Parts
510 Bailey Avenue, PO Box 350
New Hampton, IA 50659
800-431-8616
http://www.trimarkcorp.com/en/index.aspx
If you have all the parts you can put it back together. What generally happens is one of the pins on the hand latch will work itself out and then the handle falls off. You can remove the piece on the door and drive the pin back in. There is a plastic piece the pin goes through between the back plate and the handle. If the plastic part or pin is lost, call the above company for new ones. They may send the free.
Note the back of the base plate where the pin goes may need to be cleared away to expose the pin hole. Use Loctite to secure the pin.