Outside of an exchange, you can buy and sell Tezos stablecoins like USDtz and ETHtz through an independent Mintery. This is a particularly good option if you seek to purchase an amount of stablecoin that seems a bit too large for an exchange without a significant price impact.
Minteries provide a technological resource to Tezos stablecoin projects, 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 Mintery. If you would like to start a mintery, start by enrolling your institution through the 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 from/to a Mintery, contact any one of them using the links provided for each of them, respectively