Close-- the only machineguns that are transferable to non-SOTs (or government entities) are ones that were registered before May 19, 1986. Thus, there is a fixed supply. The vast majority of transferable MGs are possessed by individuals, and thus would be sold in classifieds, etc. http://www.sturmgewehr.com/
is the most notable forum for the sale of machineguns. There are also some NFA shops that have things in stock- your best bet is to google for them. If they don't have a bad reputation online and sell a fair number of MGs they are probably GTG.
You can buy from an out of state dealer but the item must be transferred to you on a Form 4 through a Colorado Class 2 or 3 SOT (ie NFA dealer such as Jensens).
If the item is on a Form 3 at an out of state dealer, there is one tax stamp involved getting it into your hands.
If the item is on a Form 4 in another state, there are two tax stamps involved.
If it's on a Form 4 in this state, just one stamp is involved and no dealer has to be involved.