The normal way is to fit the bushing to the barrel. You can buy bushings of different sizes, for which you have to measure the ID of the front of the slide, but if you can't get that exact diameter, you will be back to fitting again.
I recommend that you either talk to a gunsmith and/or get a Brownell's catalog (www.brownells.com) and check out the many bushing and barrel combinations available. I think you will want to assess your needs. The better stuff is not cheap, and you might not want to spend a lot of bucks for a minimal improvement in performance.
If the gun is not a Colt, U.S. GI, or a good quality modern pistol, you could be wasting money.
Jim