I purchased a few different opuntia pads from these folks a couple years ago. I've been vary happy with the results. I live in Seattle WA and the pads from Great Plains nursery have grown very well, blooming and multiplying many times over.
It's critical to grow Opuntia in course and well draining soil, especially in wet winter climates such as Seattle, Portland or Victoria. In late spring and into July they enjoy regular watering and high nitrogen to grow fast (assuming they're in course well draining soil).
I've been especially happy with a large pad variety originating from the mountains of New Mexico and two a plains pricky pear types, one from Little Big Horn Montana, the other a Claude Barr selection from South Dakota.