I don't recall seeing anything last time I went there. If you're climbing in the Wye Valley you're probably best off heading to Up and Under in Cardiff. About 45mins away.
Probably a bit late but definitely don't have a shop sorry to say. Up and Under in Cardiff is probably your closest decent shop. There is a GoOutdoors not too far off the M4 too.
Although there is also a Cotswold Outdoors in Brecon too