Thanks everyone!
Rick, there's one set of Microscale 48-704 decals listed in-stock at Des Plaines Hobbies. These are the older '62-'74 sawblade scheme with the large "turbo" eagle on the long hoods. If you need the later eagle+sawblade cab logo, I had to make my own for some GP15-1s before Atlas released them in Mopac. They turned out just as good as the Atlas Master MP15DC logos, and even better than the Atlas GP15 ones, so I kept my custom-painted GP15s and sold the Atlas factory painted ones.
Unfortunately I lost the cab logo artwork when my PC crashed a few years ago. But, it's not too hard to make them. I don't have an Alps printer that prints white, so I had to get crafty with a two-layer decal. (Edit: I didn't have any luck printing directly to white decal paper, but found that this 2-layer decal provided sharper results.) After getting the size and aspect right, print the attached logo or similar on clear inkjet decal paper. Seal with spray gloss clear. The white parts of the logo will wind up as clear. Then cut white decal film slightly smaller than the logo decal so that the white eagle and sawblade teeth would show through. Position the white base decal first, then carefully overlay the clear decal with the blue and red printing. I also wound up making the EMD builders plates on clear decal paper too since they go over the white sill stripe:
