That is a tough question to answer. I am in the process of testing different brands for accuracy. I think the grouping technique may be less useful than simply using a shooting chronograph, but until I research them enough and buy one, I have to stick with grouping. Most brand name ammo is fairly accurate but so far I am happiest with Black Hills. But they may be overpriced. I will eventually do my own research, but in the meantime the guys at Sniper Central did some research.
For people stopping power using a pistol, there is so much debate on different ammo it is hard to sift through it all. I don't get enough opportunities to shoot people with pistol ammo under various circumstances and ranges to be able to side by side compare different projectiles. The caliber, load, range, number of rounds, locations hit and even the clothing of the person you hit can all effect a rounds' stopping or killing power. I have gained as much info as I can from as many shootings as I can. The info more often than not comes from police shootings. Even with that, there is a lot of mixed info out there. Some love various rounds, some hate it. The intensity of a good pistol fight is such that it only takes one good or bad experience to make most people strongly believe in a round's ability or lack thereof for life. However I have yet to find anyone that was dissapointed in Speer Gold Dot. So until I get more info, that is my round of choice, particlularly for 9mm, however if you can get a bigger round than 9mm, it will not hurt. Ball can still kill people, but even 1 full second for someone to crumple can feel like an eternity and jacketed hollow points are undisputedly superior to ball when it comes to ending your enemy's life with the speed you want.
The answer to that is both simple and complicated. You can go to AmmoSeek and get a list of all the places that sell ammo online (that is the simple part) - it is listed in price per round.
But shipping is not listed and it is a big part of the cost. So be careful. To make things worse, who has the best deal one week, is certainly not the person with the best deal the next week.