Ebay was the cheapest place I could find when buying some. Sites that sell soap making supplies will sell it, but prices are usually exc VAT and then the postage can be ridiculous because they usually post out bulk orders.
If you need other ingredients too though or buying lots it might work out better. The only sites I know that sell it off the top of my head are sold out atm, but The Soap Kitchen sell a product called Disodium Lauryl Sulfosuccinate which is supposed to be similar but still milder than SLS, it might be worth having a look at. I can't say I've tried it though!