The game website offers a link to buy the game via Desura - I would urge caution on buying via them as the company who owns them recently filed for bankruptcy [1]. It's unclear what will happen to the site, but it's not worth the risk when you can get it via steam!
I am sad to hear that Desura filed bankruptcy! I try to buy from GOG.com, gamersgate.com or humblebundle.com just so there is some competition in the marketplace.
1.http://gamasutra.com/view/news/245334/Desuras_parent_company...