Search on eBay might be your best bet. 99.9% Borax is what you should be looking for. Anything less than that is best avoided. 20 Mule Team seems to be the one most people recommend, however it's probably all the same stuff from the same source. ebay.com listings do not seem to list the purity for some reason. ebay.co.uk use the same brand name but do specify the purity.