Where to get bed/box parts for 57 Dodge sweptline

If it was me I'd keep an eye on the PICKUP TRUCK FORUM. I've gotten various truck parts from members there. Hemmings maybe. I haven't bought any issues of the various truck magazines but I'd guess they'd be a good source of info.

This site has a lot of links to early Mopar truck parts:

http://www.dodgeclassictrucks.com/newretroparts.html