Simplest and easiest way is use Binance exchange. Binance support USDT with ERC-20 and TRC-20 networks. I recommend trasfer Tether based on TRC-20 network because ERC-20 for big price of ETH are so expensive and slow to use for transfers. You need simply select trasfer Tether by TRC20 to TRC20 wallet addresses. You cannot move Tether between ERC20 to TRC20 wallets or from TRC20 to ERC20. Transfer only from ERC20 to ERC20 or TRC20 to TRC20 Tether wallet addresses.

Tether address format example which is based on ERC20

0xb774A0b80A2F2D27caC0D3ECb73E6Fd06049b439

Tether address format example which is based on TRC20

TYfqC8VuFLzbdpiHhMGtWEzqokAph6dTjv

In Binance you can buy easy any cryptocurrency with credit card, bank deposit and etc…