Ebay works, but they rarely have real MTRLG shirts, and when they do, they charge up the eema for it. You have to look around there and wait for a deal to come up and snurch it right away, which may take awhile.
I use celga.com to bid on T's at the Japanese Yahoo! Auctions site -
http://auctions.yahoo.co.jp There are usually cheap shirts for sale there, I got on for 100yen. Celga charges a $5 fee and then shipping, but it still came out to be only around $20 for the shirt, which is cheaper than on ebay or from MTRLG. That's what I recommend doing.
You can also bid on stuff over at the Singapore Yahoo! Auctions site -
http://sg.auctions.yahoo.com, just keep in mind that shipping will be pricy.
Ashley