Wait for the post-Christmas sales. Holland & Barrett should have some nice deals. Then again, supermarkets and Boots do their own cheap brand of vitamins. Personally I wouldn't bother with vitamin pills because your body doesn't absorb the vitamin so you just end up pissing it out. Healthy eating is what you need.
I get mine for a website called healthspan - it is a uk based site and it is a good one. I have ordered many products from there before all of which are delivered on time and it quality condition.
Wilko's do vitamins too, my mum buys her's from there - and they are cheap as chips!