Temperature tolerance is a relative term. Blue Tilapia O. aurea are the most cold tolerant, but they stop growing, just like most tilapia, when temps are below 70 and they tend to get sick. They can stand 40F for some days, but that is about it. If you can't keep water reliably near 80F, tilapia are probably not your best choice.

I only know of one supplier of them in small quantities.
http://www.miami-aquaculture.com/tilapia.htm