Good deal. That is a refrigerated air dryer for pneumatic controls in a commercial building. That is also an older one that uses R12 refrigerant. The air should come out the side that has the drain or tank you see. If you open that up you should find a filter and an automatic drain. The outer cover should unscrew and the drain will be in the bottom of the cover and the filter will be at the top attached to the filter assembly. The filter is to catch any oil that is pumped out of your compressor. Parts should not be a problem either because at one time there were probably a million of them in use. You need to find what cfm your tool uses because that looks like a 10 scfm (standard cfm) or a 15 scfm max. Here are some parts sources that sell to the public.
https://www.motionindustries.com/mot...tion/browse/en
http://www.grainger.com/search?searc...r+dryer+filter
http://www.mcmaster.com/#compressed-air-dryers/=qf76w3
Here is another filter that I hear good things about.
http://www.wixfilters.com/Products.aspx?ct=pac