Debrajeanette, I usually divide mine in late summer / early fall, but you can also plant in spring. This is a good guide from the UoM:

http://www.extension.umn.edu/distribution/horticulture/M1267.html

The most economical way to 'get' them is to find someone who is dividing a large clump. Otherwise, look for sales at the nurseries or buy from reputable dealers online (I recommend checking out Garden Watch or The Scoop on... for reviews)

Gary