I've bought lots of XM109 5.56mm and 7.62x39 and 7.62x54 ammo on line. 9mm and .357 as well.
I usually can find cheap .22 LR at the local Walmart stores. Just bought 2,000 rounds of Winchester 1,300 fps 40 grain at 8 cents per round there, along with 100 rounds of 12 gauge #4 buckshot.
Buying online is often a good alternative as long as the shipping charges don't offset the savings. About a month ago at Walmart, I picked up six 100 round boxes of brass case, re-loadable boxer primed 62 grain 5.56mm for 27 cents per round. That's as cheap as the Russian Tula steel case .223 found on online bulk seller sites and I didn't have to pay shipping. So, don't forget the Walmart....