I have been known to buy by the case (5,000 rounds). You save money, but that is a lot of ammo and you better want it. You need to know that it shoots okay in your guns first or take a chance.
For what they sell, Walmart has the cheapest prices. My choice of the stuff they offer are the Federal Champion 510's packaged in 50 round boxes. ($1.28 each) You will be hard pressed to get much cheaper even in case lots.
CCI 100 round boxes are good too if you like to shoot them. They are just a bit more expensive (like 3x).
If you haven't, try out all of the cheaper stuff that Walmart offers in your guns. See how it does and take notes. When stuff goes on sale, buy a bunch of the stuff that your guns like.
For general shooting (ie plinking) I use whatever is cheap and deal with the fail to fires and so forth. Target shooting is another matter and competition is a completely different matter.
In general, standard velocity 22 ammo will perform better for target shooting than hi velocity stuff. In general, the more expensive target ammo will out perform lesser priced ammo, but you pay for that 0.1 or 0.2" reduction in group size. Just depends on what it is worth to you.
Places to shop online: Champion Shooters Supply, Cheaper than Dirt, Midway, Natchez, Sportsman's Guide. There are others, but those are the ones I check first.