that's a tough one. You need to search and look up on e-bay (or where ever else) for akai mk20 tapes, if i'm not mistaken. I saw an auction while ago on e-bay, somebody was sellin the unit, stand and some tapes.
I've found the ended auctions, here's link: http://cgi.ebay.com/AKAI-MK20-12-Track-Recording-Tape_W0QQitemZ330036521936
I don't think those tapes are being made anywhere. Used and old stock is the only option.
good luck
