I have 4 girls (will add perhaps 4 more?) in a 70L tank, the boys currently have 12L each (will be dividing another 70L tank to give them a bit more room) and some of the offspring from the spawn are still in 5.1L tubs until they find new homes
I know it doesn't sound like much, but I do try to provide them with a lot of space based on my uni budget. My fish are also heated during the cooler months- fortunately its warm in Townsville for most of the year, so I save a bit of power!
All of the fish I've bought have come from pet/fish stores. Most have come from a local specialty aquarium store- they get a lot of gorgeous halfmoons, giants, deltas, doubletails etc in on a fairly regular basis. They also get other awesome things like freshwater stingrays, amongst others
The types of bettas I've bought from local stores include a black devil crowntail (bought in brisbane), mustard gas crowntail, black orchid super delta, halfmoon female, red/black butterfly doubletail female, yellow/chocolate crowntail, a very 'clean' yellow cambodian vieltail female.... and now plakat and male doubletails. I don't know if I just get lucky in finding different types of bettas? I know I'm certainly in luck with this local fish store- though it means I spend a bit of cash haha
I should add- I know it doesn't make a difference at all, but I absolutely refuse to buy anything from stores that keep their animals in poor conditions- whether hermit crabs, fish, puppies, kittens, or reptiles- if they mistreat the animals/don't keep them clean etc I'm not going to support them! So there's only two stores up here I buy fish from, purely because of this reason