You can't buy Tether from Tether, their their banking relationships are... tenuous. That should tell you something.

You can wire money to Kraken and buy Tether directly there. That's the only exchange I know of that has a working USDT-USD trading pair.

Buy why would you want to? There are many threads on this forum addressing the reliability and safety of Tether. Suggest you read some of these before you commit.