I use Illustrator (9.0, but older versions work fine), and start with the downloadable vectorial drawings from Airbus's website, for the most part. They're quite flat, so add some gradient shading to simulate shadows and you're off - your imagination is the limit. Then I export them as JPGs, usually tweaking a bit in Photoshop, but that's optional. The 777 base came out of a technical PDF document I found on Boeing's website, and required retracing - fairly laborious. But if you don't mind sticking to Airbus it's easy.
If you feel like spending money www.onemileup.com appears to sell many vectorial drawings, but others on this forum haven't liked them, though I've never tried.