A little more info would help.
1. Why do you think you need a replacement?
2. Is the current one damaged / cracked?
3. Does it not play in tune?
4. If it doesn't play in tune, what is the end to end length in millimeters? 65 or 66 mm is kind of the standard, but there is some variance there.
I wouldn't worry about the mouthpiece fitting. That is pretty standard unless your mouthpiece has a damaged end. Fit to the upper barrel might possibly be an issue, but any competent tech should be able to fix that, as it is a fairly minor adjustment.