I recently purchased Optunia Polyacantha, Optunia Ymochila and Euphorbia Myrsinites at Rice Creek Gardens http://ricecreekgardens.com/
Rice Creek has a wide variety of succulents.

You can order Opuntia fragilis and O. Humifusa from Prairie Moon Nursery

I have also purchased succulents from http://www.tangletowngardens.com/

I haven't been there yet, but Highland Nursery has been reported to have a good variety of succulents as well.

I would be surprised if Opuntia and Euphorbias would show above the snow for winter interest. Opuntia survive the cold winter by loosing the water in them and end up looking dead in the spring until they have re-hydrated themselves.

I am a big fan of succulents.

Here is a link that might be useful: Prairie Moon Nursery

IMO, the best non-conifer evergreens for winter interest (that are hardy/marginally hardy in the twin cities) are boxwood, holly, and euonymus.

Other (non-evergreen) grasses such as northern lights grass, and pampass grass have great winter interest as well.