Hi and welcome to the forum.
I don't have any leads on Betta imbellis, but I do know where there are a lot of Betta smaragdina if you aren't set on imbellis. One of the members of CBS has a whole bunch of smaragdina fry right now.
IBC Executive Board Member-at-large, IBC Judging Board Liaison & Registrar, IBC Certified Judge
Member of the California Betta Society
Aquabid user: Bulldogbettas