Little bridesmaids' dresses can be picked up very cheaply in department stores or even supermarkets!  A little hand tied posy, a flower for their hair, a pair of frilly socks and they are done.

Older bridesmaids are more difficult especially if you have more than one.  Department stores are probably the best bet for dresses (BHS and Debenhams are pretty good).  These stores often have a discount if you take out one of their store cards (but be sure to pay it off straight away as the interest is horrendous). They tend to clear their season's stock in the autumn, so this is a good time to pick up bargains.

For bridesmaids' tiaras try Weddingbitz.com for some reasonably priced ones.

Make sure the dresses come right down to the floor, so the bridesmaids can wear their own shoes and you don't have to fork out for matching ones.

If you only need a single dress, ebay can also be a good source, and it is also good for pageboys outfits.