The alphabet beads I have used the the project below were a present which I believe came from Cox and Cox http://www.coxandcox.co.uk/products/alpha-bead-kit-60gm3mtr-cord and I think they were quite expensive at £6.
However I have done a little research for you on e-bay. There are lots of alphabet bead sets, but also it is possible to buy sets of 50 single vowels like a's and e's, which is brilliant as they are the first letters to get used up. So what I would suggest, if you are making bracelets etc. for Brownie groups would buy a couple of alphabet sets plus a extra set of each vowel - the shop below has sets of 50 single letters for 90p + 90p P&P
http://stores.ebay.co.uk/Angs-Attic/Alphabet-Beads-by-the-letter-/_i.html?rt=nc&_fsub=2079338015&_sid=76809655&_trksid=p4634.c0.m14.l1581&_pgn=2 - and I don't think it would take much to put together a matching set. Hope this top tip is of help!!