Where can I get an air compressor that can blow air (not just inflate tires)?
Here is a suggestion that may make some sense on several levels. Buy an air tank. I have a moderately big one, that holds 10 gallons of air. Of course, it is heavy to carry around, being steel. But when I need to pump up a tire beyond the range of the several hundred feet of hose I have, or for whatever use I need, I just disconnect it from my compressor lines where I have it coupled with a quick disconnect.

In your case, my suggestion is to pump it up with a foot pump. :) Ok, yes, this will take some work, but once pumped up, it will keep that air under pressure until you want it. And since you are looking for a "green" solution, this is as green as you can imagine, using foot power. Or, I suppose you could always just go to the local service station and pump it up to 100 PSI or so. Once the valve is shut, it will maintain that pressure for a long time.

Fix up a short length of air hose so you can use it to blow air. One of those plastic spiral air hoses might be perfect, allowing you about 20 feet of walking distance.

An even cheaper solution is to just buy a truck tire inner tube. Pep Boys sells them for about $15. Go to the gas station to pump it up, and then you can carry it anywhere you want, for a ready source of compressed air that weighs very little to carry around. Again, you will need to figure out how to put a line on the tire valve, but this is not hard to do. Go to your local hardware store and they should be able to get you set up. (I did not say to go to Lowes or Home Depot, but to go to a store where they actually understand hardware.)

EDIT:

Then here is what I'd do. The first part will be to put a hose onto an inner tube. I don't know what thread an inner tube valve is, so I would go to my local hardware store and get them to help me find a nipple with the proper thread. They tend to stock entire shelves of them, so this is not a real problem. Once you have that, you need to be able to couple it to a 1/4 inch NPT (National Pipe Thread.) This is a standard thread for air hoses and the like.  When you add the coupling to the tire valve, you will need to remove the needle valve inside. This will allow air to go freely in and out of the tire tube.

Next, add an Air line ball valve. This will let you shut off the air positively, because you want it to stay pumped up. Notice that the threads on the ball vavle are 1/4 inch NPT, exactly what you need.



Next, I went to Amazon and typed in "coiled air hose" into the search field. Typically, these run from $10 - $30, depending on the length of the hose. I find they work nicely, giving me an extensible hose. You don't need a long one though.



Finally, I looked online for a quick connect coupler. Central Pneumatic or Campbell Hausfeld  are common names here. The quick connect coupler will allow you to easily attach a handheld air blow gun as you show. Note that it will have a 1/4 inch NPT thread! You can also attach a regular tire valve, to use that to blow up your reserve of compressed air, using either foot pump or a gas station pump.



All of these items are common things you will find in any local hardware store, or even most auto parts stores.

Of course, the amount of air you can put into such a tube is somewhat limited, but it will easily be sufficient to keep a keyboard clean. You can put far more into a steel air tank, because you can store higher pressures there.