Depends what you are after ? Smart evening, smart daytime, beach shorts..
Calangute has the usual 'designer' shops - Reebok/Adidas/LaCoste etc. Not a massive choice but always something in, I wouldn't say they were any cheaper than you can buy in UK either.
Candolim has the Acron shop which is on the Fort Aguada road heading towards Sinquerim on the right hand side, I guess about a 5 min stroll from Ruffles bar. Bought some nice ones from here for 550rps last year, very good quality, a bit further on is a decent shop that sells genuine brands (think he may be linked to the shop on Calangute high street that sells football shirts ?)
Plenty of other shops and road side stalls around with something on offer, quite a few fakes also so don't go paying top prices.

Thanks for the reply Spike
Danny