As you are most likely aware it is illegal to sell wild game in Michigan and most other states as far as i know. That leaves you the task of finding domestic Caribou which would most likely be raised in the Artic areas. I have not personally run into any websites that feature domestic Caribou.
If I was you I would get a Red Deer or Elk roast. They are raised commercially, and are similar in size to Caribou. Just do a search of "Red Deer" or "Domestic Elk" and you should be able to find sources.
Dave