Check this site:
http://www.barkindogclothes.com/ Rena does a GREAT job! Prices are very reasonable and shipping is fast. Most of the tanks listed have sayings on them but if you email Rena ahead of time and ask, she's very prompt in answering and will let you know what to do when you order to ensure you get plain tanks. There are other sites so if you haven't done a search here, try the search link - several on here have their own sites and I'm sure they'd be more than happy to help you
Good luck in your quest for the tanks!