When they're raised together they figure out the pecking order very early and don't harm each other as adults (although they do still compete, but it's very subtle).
When two strange females are introduced, they will fight, sometimes severely. The territorial instincts are very strong for Bettas.
I've had females (breeders) meaner than males could hope to be in the spawning tank... ripping off strips of the boys fins and slurping them down like spaghetti noodles. It's a tragedy when they "noodles" look like this:
(same line from the same breeder, but a different individual pictured due to my old laptop waiting for data recovery indefinitely)
View attachment 3884031
Where was I? Oh right. You can buy female Bettas from established sororities and they will be fine. If they're in the same tank at a store, or if you don't have a good store you can order them online, there's at least one site I know of that groups the females individual listings by what tank they're from.
Mother won't let me import any more $100+ Bettas due to the aforementioned breeding attempts and living in an RV.