What's involved in replacing a 6 ft bed on this truck, and where is the best (least expensive) place to find the right one?

 

it should be fairly easy unless one of the lower clips break. intead of using nuts on the bottom of the bolts there are threaded clips clipped on th the frame. just remove the tail lights if they are tstill good. then remove the harness that connects to each tail light. remove the bolts and find a couple of friends to help you switch the boxes. when you get the new one on you will need to line it up to get it straight, the factory uses a tool that sits on each side of the rear bumper then two people pull it back to the tool to get it straight with the bumper then look up the side to check how it lines up with the cab. (i work at one of the ranger plants and have done this job) mostly just make sure its straight and tighten the bolts hook up the wires and go.

your cheapest but would be a junkyard for a box. if you go to a junk yard either bring your truck without the box (tie your lights to the bumper) and have them help you set it on the frame and put a few bolts in to get home or see if they will take the old one and prep it before you go so it wont take long to do the switch.
or find a trailer big enough to carry it.

another way you might find one is if you see a company with contractor boxes on f150s call them and see if they still have the boxes laying around. a guy i work with got one for his 88 that way. mint cond. never used. they bought the truck took it off and set it aside.