Hi, Robert. There are many places around the country that buy old buildings, strip them of architectural features and then resell those. I know that a large one is in Chicago and many are on the web. (Try searching for "architectural artifacts" or "architectural antiques" or other variations that you can come up with.)
If you are looking for an exact match, you can take a photo of the handles with a ruler for scale, scan in the photo and send it to the dealers with your request. Realistically, some are pretty good about trying to fill small requests and some are not.
Every town has one plumbing supply place that specializes in old plumbing parts and every town has a place that specializes in upscale plumbing fixtures. You might try your nearest plumbing supply store and if they can't help you, just ask who might carry something like that. If you do some looking, I am sure that you will find what you are looking for, but I doubt that you will have much luck at the big boxes.