How to best find the right Warrant Officer for a letter of recommendation, if none are in my unit?

First if you are in a BCT then you should have a BAO in your Brigade HQs. They are the Brigade Aviation Officer who is the liaison for operations between the BCT and the CAB. If your Brigade does not have one then you need to make a connection with the Command Chief Warrant Officer of the CAB that is on your post. I know some Officers who want to have a chance to sit down with a NCO before recommending them. Bring your personnel files with you in case they want to review it. Also, going to outlook and sending an introductory e-mail as suggested works.

All else fails do exactly what everyone is telling you - head over to the airfield and seek out help. Again, going to the BDE HQs and setting up an appointment with the Command Chief Warrant Officer is about the best letter of recommendation you can get. There are also other CW5s at the brigade level. 

Good Luck and study for the tests. Ensure your records are squared away and if you don't get selected the first time keep trying. I have seen several selected later.