Buy your normal size pre pregnancy, so I wear a size 16 maternity.
Buy what you need. Trousers, tops, need measured for bras when your boobs go up. Personally I invested in some size 18-20 seam free knickers from Primark as I'm an over the bump girl and I like my BIG pants.
Bump bands - you can get 2 for £6 from Asda, You can use them to hold up mat clothes that just needs a bit of holding up because you're not quite into them yet or cover up any gaps your tee shirts/tops may leave if they ride up.
Where? I got a good load of stuff from ebay - keeps the costs down, trousers from Next that normally cost £18 I got for £5 incl p&p.
Primark - I just bought some stretchy tee shirts to show off my bump and some tops a size bigger than I normally wear and touch wood, so far been fine. I also bought some cheapy leggings as they are comfy for lazing about in and don't constrict my bump.