As a last resort, you can figure out the inner diameter by doing a bit of simple high school math. If you are talking about machining your own, I'm sure you already know how, but see the link below if you need a reminder.

See:

http://www.405th.com/showthread.php/...r-Tutorial-V-2
Method 1 is the one to look at. The tutorial was originally meant for costuming/cosplay, but the math works well for anything where you need to change the scale of something.