Enter Amazon, which is selling things like this DVD of Archie Comics for super cheap. For four bucks, you can get 120 digital comics. That's an amazing deal, and while there aren't a lot of those to go around, there are definitely other digital collections you can purchase for much cheaper than actually buying them piecemeal.
Turning those comics into PDFs (or just finding them on the disc) is usually not a problem, and then just dragging the PDFs into iTunes and cracking them open in iBooks is even simpler. And voila, you've got tons and tons of comics to read on your iPad. Excellent tip, and a great way to pick up a lot more comic books to read on your next plane trip.