I'm trying to paint my plane in RB3D and I can't find the location of the .bmp files, it's not in the paint folder or anything, yet anytime I open paint shop, they are apparently there
(unchanged of course)

The actual files are in vol files. However you can export the files from Paint Shop, paint them, and the import them. So look for the Export / Import buttons. The files will be then found in the Paint folder.
The files need to be in 256 bit color.. Pure black is an invisible color so your crosses need to be an off black or else you will have cross shaped holes in your wings.
You can get the default paints for MP or a very nice set of skins for MP from the the western Front Mod here: http://www.flybert.net/mpdefalt.htm
Since these are originally for MP default, the files of one will rewrite the other. But between the two you will get a very good collection of paints to mix, match, and modify.
03240985.bmp is how the default files are named. the 03 means it is an airplane, the 24 what type of aeroplane (in this case a Sopwith Camel), the remainder what part of the airplane (this case an elevator. Below is a list to help you find the images you are looking for:
Fokker E III, 0300
Albatros D2, 0304
Halberstdat D2, 0305
Albatros D3, 0306
Albatros D5, 0308
Fokker Dr1, 030a
Pfalz D3, 030c
Fokker D7, 030f
Pfalz D12, 0310
Morane Bullet, 0313
Nieuport11, 0314
Nieuport 17, 0315
SPAD 7, 0317
Nieuport 24, 0318
SPAD 13, 031b
NNieuport 28, 031c
DH2, 031e
Sopwith Pup, 0321
Sopwith Triplane, 0322
Sopwith Camel, 0324
SE5,0325
Sopwith Snipe, 0328
Hope this helps

well that will when I want to do that,
But I was talking about the files you export from the paintshop. I know of course that they'd usually be found in the paint folder. But any files I export don't show up on my pc (at least in the normal places) but somehow RB3D can find them (unaltered of course)
I think it may have something to do with an earlier version of the game I had installed on my PC, but I have deleted any obivous files (like folders titled SIERRA) but the GOT version is still picking up files from somewhere, like paint files that I've tried to export of course....but also saved games.
which on the one hand is good, but maddening cause that means there are more files that I can't find.
where else would RB3D store files?

I just tried it and the image is being exported into the Paint folder of the GoG directory. If for some reason it isn't. Export a file from Paint shop in an easy yet distinctive name, and then do a search for it.
If yopu exported a fuselage image as FUS, then search for Fus.bmp and see where it ended up at.