No disrespect to Mr. T, but in addition, Dave Corbin, swage.com and other sites, has a directory of many, many bullet makers that lists the diameter bullets they make. (Of course he hopes they use Corbin equipment... of course.) If you visit his site and roam a bit this should come up and if you wish you could purchase the book in hard copy...
I am guessing you have a .38/55 or .38/56 from yesteryear. Great medium bores at close range. And you slugged your barrel... Years ago I had a price list from Barnes, before Fred sold out, and he had all sorts of odd diameters and several jacket thicknesses to adapt to the gun firing and the target. You might try the Barnes factory. Don't know anyone who stocks everything. LUCK. Happy trails.