I'm in a zone 6/7 and have good luck with thornless blackberries. Below is a link to the U of Arkansas which have done a lot of research on those.

If you need many plants, find a vendor who has root cuttings, as they are far more economical than plants.
they are pencil-sized roots that you plant in the field in very early spring, so you need to source them now, otherwise they sell out. Try several varieties to extend the season, build a sturdy trellis, mulch well.

I recommend Enoch's, good plants and he will talk with you anytime:
http://www.berryfarm.com/

Here is a link that might be useful: U of Arkanss blackberries