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.

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.

For  cheap tiaras for bridesmaids try Weddingbitz, they also have  bridal combs and hair bands which would be suitable for bridesmaids.

If you only need a single dress, ebay can also be a good source, and it is also good for pageboys outfits. If you have several bridesmaid it can be difficult to find several dresses in the same style and in the correct sizes on ebay but I recently attended a wedding where the bride has neatly solved the problem.  Her 3 adult bridesmaid wore the same style of dress (a popular strapless satin design) but in different colours (burgundy, pink and lilac).  They carried posies of sweetpeas (grown by her uncle) which included all three colours mixed together and this co-ordinated the theme.  The effect was sophisticated and very effective (and the dresses only cost £25 each!)

Wedding Bitz for Bridesmaids' Accessories