You can buy photoshop from various computer related retailers. Amazon should have it aswell although i have not checked. You can buy it on its own, or if you would like to do other creative media projects you can buy it as part of a creative suite. The most recent one is CS4 and there are various versions of it but check that it includes photoshop.
Its not cheap to buy but its used by proffessionals so you know its worth what you are paying the money for.
There is a free 30 day trial version i believe or if you have no problem with downloading illegal copies then there should be versions available via bittorrent sites. Just be aware that there will be some fake ones around, i would recommend looking at comments and ratings as other users are pretty good at spotting fakes.
