you can use Bitquery solana APIs. You can create a free developer account and use it. Here's the link for the saved query to get SOL/USDT price. But currently Bitquery is only providing realtime data. They are adding historical data to their database too. So keep an eye out on their telegram channel and Official Twitter account to receive the updates.
You can create a free developer account and use it.
Full disclosure: I work at Bitquery. Feel free to ask questions in our support channels.