In the Pta area your best bet is probably Hikers Paradise in Centurion (http://www.hikersparadise.co.za/jsp/index.jsp
). If you're willing to travel a bit farther afield though, Drifters in Sandton City mall (http://www.drifters.co.za/index.asp?pgid=58
) offer a sweet selection in gear. If I were you I'd compare prices for the shoe you want between those two stores and then check http://www.mountainmailorder.co.za
as well. Sometimes the difference in price is quite big.
EDIT: Oh yeah, also check out the Climbing Barn in Pta East, way out on Lynnwood (http://www.climbingbarn.com/shoes.html
) as they also have a gear shop. And then of course Wonderwall in Kya Sands, Joburg also sells gear.