I would check the Athearn website, they have a pretty extensive list of parts. You could always drop them an e-mail.
Another alternative would be to track down a dummy GP-9 at the LHS, online, or at a flea market. It comes with all the 'detail' parts you'd need, and won't cost too much...in fact, if you find a good bargain at a flea market, it might be cheaper to buy the dummy GP than to order just the detail parts from Athearn, once you factor in shipping !!
Of course, once you buy replacement parts or a dummy, you will find the original parts in the tan envelope...that's just how life works !!
Hi guys (and gals, if there are any, currently)...some may recall I started a thread called: "She's Real Fine, My GP9"
And I posted a shot of it, on a diorama I had made to take such photos. Only, when asked who MADE that lovely engine, I had no idea...as I was then in Palm Springs for the winter. I said that I'd run inside the house once I got back to B.C. (Canada) and downstairs to find it, and report back to yooze guyz, on just what it WAS.
Well, I've been home now for over 6 weeks, and finally got to where I could stop working and doing personal chores long enough to sit at my little train workbench in the basement, and examine it.
Turns out, and I'm quite surprised, that it is an ATHEARN model. I found the "blue box" it came in, and dug around to see where the handrails and such extra stuff that should have come with it, were...not in that box, I realized...I left here to go south, leaving a giant mess of stuff strewn EVERYwhere, so I can only hope that among all that junk, is a little tan envelope containing the detail pieces. I do not think it would be all that easy to find REPLACMENTS for them.
By the way, I'd mentioned, I think, that this engine operated with terrific smoothness, the few times I've been able to run it...taking off with a barely discernable crawl, no problems accelerating realistically. I had a boxcar running behind it with old Soundtraxx equipment in it, leading not only to a decent speaker in the freight car itself, but also to two gigantic speakers on the floor, resulting in a window rattling diesel noise. Impressed the heck out of visitors and the neighbours, anyway--none of whom had ever seen or heard SOUND with a model train.