I imagine your local firearms dealer would be able to order one for you, but he can only order a semi-automatic version.
If you're looking to get a fully automatic Tar-21 assault rifle, you need to file for your Class III permit, which is required to own automatic weapons, suppressors, and sawed-off shotguns. This can be done by paying the $200 tax that is required and getting a background check.
It's going to be expensive (most likely over a thousand) and it will take time, but talk to your local gun dealer.