There is a field near where my in-laws live in South Wales, that is inhabited by a small herd of what look like pretty good quality Sec Ds. There is a sign up permanently saying "Welsh Cobs For Sale" and they do seem to be reasonable horses. I'm not sure I'd buy one from the field, but given that the sign has been up for at least ten years and the mix of horses changes quite frequently, someone must be doing a fairly good trade in them.
You could try the rotherdale stud: http://rotherdalestud.tripod.com/
They are only just inside the welsh border. They own Tydfil Magic princess who won the royal welsh 4 times and her daughter rotherdale magic princess who is a world champ and unbeaten as a foal. My boy is from there: (not very good pics sorry very old) web pageweb page
He's 9 now, owned him since 2yrs, wouldn't be without him ever!! He's wonderful, typical section d full of personality and quirks but so wonderful, kind, excell all rounder etc etc! They are really nice at the stud too, have written to them a few times. The one for sale on there at the mo has the same sire as mine. I would get intouch though, don't think they update the web site that much. There are hundreds of sec d studs though, just they're my only experierence.