I suggest that you ask this on the Cactus and succulent discussion forum. You will probably get more reactions. People don't come here as much. Prickly pears are used as landscape . check out . They have a lot of varieties and will be open about the Zones that they are hardy too.

http://coldhardycactus.com/

Leo Chance has writen a great book about all the cactus that are available for cold places.

http://www.amazon.com/Cacti-Succulents-Cold-Climates-Outstanding/dp/1604692642

Intermountain Cactus nursery lists their cactus by Zone so one can know what will work for you.
http://www.intermountaincactus.com/Intermountain_Cactus/Home.html

Here is a link that might be useful: Cold hardy cactus Nursery.