Does anyone know how I can get a plastic, drop in bed liner for a 1975 F250 crew cab, short bed?

 

Goto your local truck accessory store. Aftermarket has about anything you want available these days.

 

Why bother with slippery, moisture trapping, rust causing plastic liners when do it yourself roll in liner kits are available cheap at most auto parts stores

 

I agree with western star. I put a $80 Herculiner kit on my trucks and a few of my friends, and it holds up great to all of them.

But if you want a plastic liner, then go to a junkyard. Just about any full size truck with a short bed will work.
I've seen 90s chevy liners fit in 70s ford beds before with zero modification.