I don't know of anyone that sells the plain Custom trim level panel on the aftermarket (i.e. re-pops) - you can probably find them on this board. LMC, and many others, sell the correct (no woodgrain) scroll panels for your 72 though. Just don't like the scroll?? You could probably sell your scroll ones on here for less than you would buy your non-scroll ones on here
Check the parts board for folks "parting" entire trucks. If they are just the Custom level, well, there you go! Probably get em for a song since most prefer the scroll. Personally, I like the scroll ones, think they are a nice touch, but completely understand wanting to have your truck the way you like it. (on second thought, you might want to hang onto those original scroll panels in case you ever sell the truck - the seller just might want em...)