I have used all 3.
Sale barns can work. Try not to buy singles or pairs, as these are generally someone else's problem they are trying to get rid of. Try to find someone who is selling his entire herd, or a certain class of cattle. We have a local guy who retains his own females, and sells every cow once they reach 7 years old. He leaves some good years on the table, and someone gets some purty good cows.
Private treaty can work, but one of my worst experiences came that way. Bought 12 bred cows from a fella about 7 years ago, and found out about a month later this guy had just bought them from the Sand Hills of Nebraska just a week before. They looked good, but that was about it. They were not the age they were supposed to be, not bred to what they were supposed to be, and several of them were just down right mean. Had two just up and die the first year, one from trying to have too big of a calf, (nice surprise that morning) and the second just found dead for no apparent reason. 3 tried to kill me during spring vaccinations. After two breeding seasons, I only had one left, and still have her. In hindsight, I should have done some more research and I would have known this guy trades cattle every now and then.
You can find good affordable cattle at any venue, you just have to be very careful and watch what you are doing.
Ranch sales are normally good quality, but you sometimes you end up paying too much.