For the adventurous and computer/design savvy, you could try some DIY decals.
I've been in contact with
QP Media, (must get around to ordering) who sell printable adhesive vinyl - you could try your hand at printing your own. The only downside is that your average inkjet doesn't print white (for clear media) and doesn't pre-cut the vinyl (for glossy white media). I'd recommend pigment inkjets over dye inkjets, for reasons of light-fastness.
I plan to do some on clear media, printed mirror image for application inside the shell, from signatures and graphics I've Traced into a vector graphic using Inkscape. As I don't have an airbrush, this may be the best way to do really eye-catching shells
- I could even use photographs.