Dealer wants $15 for small plug and is a prepurchased part only. May call different dealer as well.
May do LEDs if nothing else.
I got a replacement from Parts Guy Ed website. Look under "rear lamps->side marker lamps". There's the socket back part (that plugs into the wiring harness), a front part (either amber or red) and then if you need it, you can also get the metal retainer clip.
Turns out I didn't really need the clip after all... I thought mine was missing on my new-to-me truck, but it had just slipped back and was still there hanging on the wiring. Oh well, now I have a replacement just in case.
Installing was relatively easy for me since a bunch of the plastic clips had already come loose from the cover, so I could reach a hand back. Unfortuantely the loose cover meant water sprayed back onto the red one and now I discovered that it's socket is all corroded so I'll need to get a new one of those.
Picked up some replacement plastic clips at the local auto store and good to go. By the way, those new retainer clips can be really tight to squeeze on so you might want to fuss with it a bit ahead of time and see if you can do it first before using it to clip the socket assembly into the dually fender.