http://www.streetarticles.com/fish/green-bettas-colors-and-genetics
Sounds like they're not all that easy to come by.
Fish with more black will have a darker, more pronounced color of green in their scales and fins. Unfortunately, this type of green is very rare and difficult to breed well. For this reason, these fish are typically bought up quickly and a hefty price is charged.
Sounds like they're not all that easy to come by.