To make your life a little easier I have done some searching for you and here arc some on the best offers on the web (see you don't even have to leave the house)
Try this Instant Cat Set from Fancy Dress Outfitters for just £3.42. It contains ears, tail and bow tie you just need a black outfit underneath.
a2z Kids have a great selection of costumes including this Spider Witch costume for £3.95
Silly Jokes has a great collection of Halloween Costumes. For girls they have this Story Book Witch Costume for just £4.50
If you are looking for something a little different for you baby then check out Baba + Boo's Halloween collection.
Here is BG modelling their Halloween Hairband £3.50
Of course there is always the supermarkets:
ASDA have these great Skeleton and Witches costumes for £5
If you are looking for something different then check out this round up on All About The Boys
Whatever costume you choose have a great time!