If you're only interested in saving money you can do that by watching the newspaper ads........but if you want to buy some quality beef then listen up.
First mistake, you're not going to be shopping for a cow, you're looking for a steer. Cows that are sold on the beef market are usually too old to bear calves or to produce milk (or something else is wrong with them), that's why they are on the market. What you want is a steer. That's what is raised for meat. Do a google search if you don't know what a steer is.
Next, ask some friends and co-workers about farmers in your area that sell their cattle for beef. Once you have found that farmer ask for details about how he will prepare the steer for slaughter. In my opinion beef that is taken straight from the pasture to the butcher isn't that great, but steers that have been penned up and fed grain so that they have gained some fat make the best eating. Fat is where the flavor is when you're talking beef, that's why Japanese Kobe beef is so highly regarded and so highly priced.
Finally, once you've found your steer, and had it properly fed up, the next thing you need to find is a butcher shop or slaughter house that will not only cut the meat to your specifications, but will also return the same beef to you that you delivered to them. So many meat cutters buy cheap, underfed steers to put for sale in their own meat case, and can't resist the temptation to switch theirs for yours.......and that's exactly why I stopped buying whole beefs. Too many times I took a fattened up steer to the butcher, only to get back some very lean beef that reminded me of school lunch every time I took a bite of it.
Buying a whole beef can be tricky business........you'll either get the best meat you've ever eaten or a freezer full of dog food...good luck !