I bought several mulberry trees at about 3' off Ebay. They weren't very bushy, but a summer outside took care of that. Best of all, they were really inexpensive. You can buy seeds on Ebay as well if you have some time! Anyhow, just go to Home and Garden-Gardening and Plants- and then type in mulberry. You will find plenty to choose from, even at this time of year. Be aware.. they come as bare root, leafless sticks and part of them die.
I am not sure if bonsai would work on one or not. I plan to just.. grow mine into trees. I am leaving them in containers at least for the time being, though.