Depending on what you're looking for, and how much you're willing to spend, there are a few choices.

The major one, are decal sheets you can buy. These are typical military markings, and should be avaliable through most good LHS. I can't remeber the brand name off hand.. Parma (?) make a whole line for cars, and I know they have one or two that are suitable for planes.. You can also contact the various manufactureres to sell a sheet, from one of their planes...

You can buy sheets of 'water slide' decal paper to put through your printer. so you can make your own with your computer (jsut need to fuel proof them afterwards..)

You can always cut 'Monokote' and assemble the shapes / insignia as you want, then just iron it on.

You can approach a sign maker, and have them make you a set of vinyl lables, like what the signs are made of. Speak to them, and most of them can take Corel, Illustrator, or DXF files. Also ask them for their 'cutt off's as you can use them as trim sheets! or cut out your own.