ebay.com definitely. In college, basically my whole chapter did this. I used to buy from an ebay seller called lillycloth. here's the seller profile:
http://myworld.ebay.com/lillycloth/
also here's a list of items she's selling:
http://shop.ebay.com/merchant/lillyc..._sopZ15QQ_scZ1
she sells squares and large rolls of fabric, so you can get just a little bit at a time which is perfect for letters. when you get the fabric, you jsut bring it to your local greek store.