hiya, how far down south are you ? im in brighton, and manage to find female bettas all around me.
the main local fish shops i use for female bettas are,
Paraquatics, in Portland Road, Hove, tel, (01273) 733 996 (this is a nice place, but be careful, they get confused about short finned males and females, but they currently have a fair few! went there 2day!)
Handcross aquatics, their found just off of the (01444) 401 004 (their the best place i go to, they know alot about them, and always have at least a dozen at a time! and their great colours!!)
All in all there are a fair few in my area that have them, but in all cases, they only ever have the normal bettas, nothing especially fancy like crown tails or anything, but they all have gorgeous colours!
i hope u manage to find a suitable female! their great lil entertainers!