WB, I create my own templates. If you would like to do the same, here’s what you do:

Take a line drawing of the aircraft (I’m providing the RV-7 here for you)
trace over in Macromedia Flash to create a vector diagram
layer with rendering and highlighting to make the drawing appear 3D
Once done, lock the shading layers and color away on the base drawing.

I’ve only worked with Flash so not sure how these instructions would translate into your favorite graphics app.

Alternatively, you could just work with MSPaint and just using this drawing play with different liveries. The final image may not look 3D and polished, but if you’re just looking to get an idea of what would look nice and don’t want to put blood and sweat into it, this is the route I would recommend.;)