I'm in the same situation too with my 40 1 1/2 ton.

I have a feeling that I'll end up making my own.

I went to a local truck shop and they wanted $4k for a standard flat bed. Way more than my truck actually cost.

I posted a topic earlier on the right size. Here's what I found out:

Dimensions are 141 1/2" X 80 3/4" for my truck which measures 84" from the cab to the center of the axle (CA) - thanks to 3B

Other bed lengths are:
60" C/A = 8' body
72" C/A = 9'-10' body
84" C/A = 12' body
102" C/A = 14' body
108" C/A = 15' body
120" c/A = 16' body

C/A = cab to axle dimension.
Standard cab clearance is 3".

thanks to 53MoneyPit

I'll be making mine fairly soon as I have to put the bed on to get a certified weight for the DMV.

I'll post pics when I'm done.