Outside of an exchange, you can buy and sell USDtz through an independent USDtz mintery. This is a particularly good option if you seek to purchase an amount of USDtz that seems a bit too large for an exchange without a significant price impact. Minteries provide a technological resource to the USD Tez project, serving as trusted validators in the minting and redemption process. These minteries are Tezos public bakeries as well, though at this time one does not need to run a Tezos baker to become a USDtz mintery. If you would like to start a mintery, start by enrolling your institution through the USDtz minting enrollment portal.
Choose a Mintery, request a minting operation for however much you would like to mint, follow the given steps.
To buy or sell USDtz from/to a Mintery, contact any one of them using the links provided for each of them, respectively