The lists I've seen certainly don't offer an option on sex, and the size they're usually for sale at they aren't sexable anyway. There is one report of breeding on Planet Catfish, so it is possible if you're willing to devote enough time, tank space and money to it. You would be best to start off with a group of six-or-so young ones to grow out to breeding size (which will take a few years), talk to your LFS and you should be able to get a good deal for buying bulk.
It is a fairly ambitious jump from breeding bristlenose though, maybe you could try some other types like Hypancistrus Spp. that are a bit smaller, faster maturing and easier to breed than Bayancistrus.