Also, ask at your local pub/s. Most pool tables are rented, when balls are worn, chipped etc they get exchanged by the machine rental company, get a couple of numbers for such rental companies and give them a ring. Now if you manage to get some and they look a little tired, put them in a dishwasher, when they come out they will be dull looking, now just polish them up with some decent polish ie duraglit, they will then come up nice & shiny. You usually also find when a set is worn out, the blacks tend not to be as bad since they don't usually get as much hammer as the colours & white do.
Failing that, try here, it's near me and they do stock some spares :-
http://www.snooker-and-pool.com/