The two MA's in Cardiff are both pretty good - the one off the M4 near Castell Coch is better than the other one in Wenvoe with regards to plants,
There's a pretty good one (normally) just off the A48 from Culverhouse Cross
MA in Bridgend is okish
There's some pretty good ones in Swansea, but I'm not really sure where they are-a Google search might tell you more
Have you tried looking at the PFK directory on their website? That's supposed to list all the aquatic shops by area-I've always found it quite reliable.