I usually use very soft brushes for cleaning the insides of my PC. Also, once in a while I do resort to my vacuum cleaner and blow air from almost 2 feet away into the insides of the cabinet. For the outsides of my cabinets, I use a very soft piece of dry cloth and clean everyday. Haven't had a problem yet with these methods.
If you want to buy compressed air cans you can order them from Newegg / Amazon (through KMD). Something like this:
Newegg.com - Memorex - Air Duster (Unscented) 1 Pack
Amazon.com: air duster
You can also use rubber blowers which you can buy locally ( I think KMD also sells these). These aren't too costly at all.