Hi and welcome to the forum.
The standard size in that ballpark would be 2" Schedule 40 pipe and can be found pretty much anywhere. Thing is... it is 2-3/8 x .154 wall...
You're looking for tubing dimensions for materials that are much more readily purchased in pipe sizes. McMasterr Carr has several different materials available in pipe size and also tubes that you could possibly machine to your dimensions? Also... If you need enough of it you can have pretty much anything made.