you can find h2o just add water on DVD at the shops but you have to buy it You can find h2o - Just Add Wather season 1, episode 2 and first 10 episodes from season 3 at: http://www.gtv33.com/2009/08/watch-h2o-just-add-wather-online.HTML
I am sorry to say that there is no h2o just add water season 4. Although there are rumors that they will start filming season 4 in November 2012. But don't take my word for it cause they might just be rumors.
Here: http://tv.blinkx.com/show/h2o-just-add-water/hOpsLDonNRq7ZwD37Ms9z1vbo4E#s2e1 hope i helped! PS: when you have finished with the 1st season on there, you can watch the 2nd season after :P anyways, again, hope i helped :)