They are not as attractive so shops don't sell them.... sometime a few get mixed in with the group by accident.... or you can find a hobbist. Also some of the better shops sell them.
The blue dwarfs are the only ones in which the females are actually blue too.
So could be there are females in the tank in the lfs, only not recognised.
Also remember most of the line bread dwarf gourami are of extremely poor health.... a by product of how they are being line bred to get certain colours.