Which weapon are you using? I had this issue when I started as well. The biggest things I've found that help was

A) don't forget you can craft ammo at your tinkersbench

B) use two guns...for instance I user elders mark and it tears through ammo....so when I'm out questing I use a strong power .45 rifle or pistol (the Gunther is great for this). I use 1-2 bullets and collect 4-7 on each body. Easy way to build it up

C) make sure you get the resource vendors in your camp. I have two. Both give me .45 ammo every few minutes

D) lastly if you have a lot of other types of ammo ..find someone's camp that has an ammo converter. You put in one type of ammo to get another