play.com or amazon are both good bets, check for the battlechest. I got lucky with mine and picked up WoW for £5 at HMV during a sale just before the battlechest came out, a friend picked up the TBC boxes for me dirt cheap in NL, but I've seen them for £10 on play.com.

Shop around, January sales might mean HMV/Virgin etc have them cheaper than online stores