I'm trying to buy some Japanese audiobooks, and iTunes seems to be the largest repository from which you can directly download. I know that jlist and nihongo pera-pera among others resell iTunes vouchers, but they charge a pretty hefty markup which I'd rather avoid paying if possible. Is there anyone in Japan with a Japanese credit/debit card and an Itunes account who'd be willing to buy them for me from the iTunes store (I'd pay, obviously)? Or does anyone know another way around the problem?

You can buy the Japanese iTunes gift cards. I don't know how hard it is to get them if you aren't in Japan, but if you're in Japan, they're sold at a lot of places.