Nile Tilapia are probably the most cold tollerant.
They are sold as live food fish here in Ohio at select grocery stores. One problem I see with going the grocery store route is that it is likely they are all males.
Here's my thread on tilapia in my Ohio pond
http://www.pondboss.com/forums/ubbthreads.php?ubb=showflat&Number=14101&fpart=1
