Bit of jackness here lads, this is simple stuff. Yes, I wouldnt carry one around day to day, as I don't want Mr St John Ambulance trying to ram one down my japs eye, but you ought to have one in training and fekkin definately on ops and you should make sure your mates know how to use it. After all, you're not going to put it in yourself, are you!
As for getting one, just blag one off your medic, they have boatloads. You ought to be issued at least one OP and NP for your kit anyway.
It takes Feck all effort on the part of your medic to teach you how to size off NP and OP airways and how/when to use them. This ought to be done.
CTC, it's the size of the gob that matters. An OP airway shouldn't go anywhere near the effin trachea (unless you're using a boot to insert it).
OP airways (also called Guedal, or 'J' airways.....cos of the shape

) come in sizes 1-4 (ish) for adults and smaller for kiddiewinkles. Most blokes are 2 or 3. Place the airway against your cheek to size it, it should go from the angle of the jaw (the point where it turns 90 degrees below your ear) to the centre of your teeth.
NP airways are urine easy to fit and most blokes are a size 7. If you have small nostrils, or are actually female, then go for a 6.
Airways are easy-peasy to learn. It's things like medicuts that are more serious (but should also be taught/learnt).