Because BE isn’t considered an “ethical” treatment, most gem traders avoid selling them and concentrate on untreated or those with “acceptable” treatments ie heat only and with emeralds - colourless oil.
If you look on eBay you can find them, mostly Thai or Indian sellers.
What I can tell you that there are stores (yeah looking at tiffany's even) that interestingly enough, they sell stones and the sales people will say "Oh, totally natural" they won't say when asked about pearls, Oh yeah they're likely bleached. They will say that Tiffany's dosen't bleach pearls. (technically true but not really)
I have no issue with BE if its disclosed to the buyerm and frankly no one else should either. Its more about disclosure and less about the actual treatment. The buyer should be able to decide if they want the stone with proper information given to them.