Source for banded agate

Hello everyone, Can anyone please give me a source for banded agate?
I am looking for some with flat, parallel bands. It usually comes a
sort of brown and white and can be dyed. I am trying to replace a
carved stone in a signet ring. I only need a square inch and 1/4 inch
thick but would buy more to have it on hand. Any lead would be
appreciated. Thank you. John