Ammo refills are extremely difficult to find. I just hit 20 hours of gameplay, have approx 1700 kills with a 2.7 kdr, and today I just found my first ammo refill in all that time, in all those kills. Since I tend to go on killstreaks often, I run out of ammo regularly and am always on the lookout for more. I play medic, as other classes I tend to have a lot of trouble finding health packs or getting healed, so this may also apply to health packs as well as ammo packs.
I'm not sure if supports are just rare in general as a class, or there's a different reason ammo crates are so difficult to find, but please consider the following:
ammo crates and health packs are visible on map. Ammo/health packs have larger and more distinct models, so they are easier to spot. A button you can press to highlight nearby/nearest ammo/health packs on your UI. More incentive to drop health packs and ammo packs. Permanent resupply/heal drops near captured points or objectives.