There are a lot of options out there. I've used this: https://www.cosumi.net/en/

The AI on that is pretty strong, at least at 9x9.

The key to winning Othello is getting the corners. Controlling the edges helps a ton in getting the corners. Avoid the 2x2 corner areas unless you know you can get the corners.

Using these tips alone should help you (easily?) beat the AI here, as it does a lot of mistakes.