I think you can get lower concentration shampoo from places like boots or your local chemist but the studies that were done were using the 2% version. It is actually sold as anti dandruff shampoo but lots of people on here recommend it. You don't use it every day, the study was done on use for 2 to 4 days a week.
I got mine off eBay, chemist 4 u, for 8 quid including postage for 100 ml. There are probably cheaper generic versions but I wasn't to bothered at that cost.