Almost any running or sports focused store...

I know that Running, Etc., in Norfolk has carried them.

or online at:

Roadrunnersports.com

paragonsports.com

Is your running apparel cotton? if so, that can contribute to the problem - cotton swells when wet, and becomes abrasive (like a drag diva tossed in a pool) - drifit, coolmax and other polyester based products are less likely to chafe. Also, loose tank tops seem to be kinder than t-shirt style tops.