Find a local motor factors and buy everything there. A lot of people buy service kits off ebay, which aren't always good quality (eg oil filters especially) and if something is wrong you can't exchange it easily.
Personally I much prefer to keep local businesses going since they are ones who you will need when you need a part in a hurry.
Don't overlook places like Halfords - some people think it's cool to rubbish them, but sometimes their prices are OK and at least they're open when everywhere else is shut. They are winding down their car servicing sales because people buy online - and I think that's a bad thing.