Your best bet would be to look at pet stores around you. If they don't have one you like, ask them when they get their betta shipments in. If you go the day they get a new shipment, you will get first picks and might find the one.
I've seen yellows at walmart as well...
Also go to the "betta classifieds" section of the forum. People always rehome bettas (and not expensive bred ones, most people rescue them, heal them & rehome them)
“Some people talk to animals. Not many listen though. That's the problem.”
― A.A. Milne
Espeon female halfmoon / 10G
Jolteon male dumbo plakat / 10G