The cheapest place I know of for quality filters is Dixons actually. They stock Hoya filters at prices that most other places don't come close too. The problem you may have is finding a Dixons branch with the ones you need in stock although you can order online if you want to.
Failing that try SRB as they stock filters for most size and fits including Cokin filter systems and Lee filters. They also make custom filters at reasonable prices and are very quick at delivering. I have used them several times and found their service to be excellent.
SRB link