Rich:
Have you looked into AeroMaster Decals? I pulled the Building The P-40 WarHawk - A Scale Modeler's Project Handbook - Glen Phillips and Kevin Hjermstad (Kalmbach Books) from the reference shelf and looked for an aftermarket source and AeroMaster Decals was the aftermarket decal reference for a P-40B build there. They used the Monogram Flying Tigers kit for the build.
If you haven't looked at this book I would advise you to pick it up. A good reference for this build.
Cheers;
Gregory