If your in the US, you can't really get one. As per ATF regulations, the receivers have to be torch cut in small chunks.
In other countries a few welds to the innards will make it a-okay but, nope, not the the good ol' US of A. :sleep
You can find parts kits which contain the leftover parts but, these are considered machine gun parts by the ATF so I would be very careful about assembling them into something gun shaped. They're sold as a novelty and are not really meant to be "built".