Whether you get your dog from a rescue, a shelter, or a breeder, you want that person's primary motivation to be the dog's well-being, not your cash. Trust your instincts. Take your time. Breeder, shelter, or rescue, if they're responsible, they'll ask you a lot of questions. (The San Francisco SPCA has some thoughts on what to expect from a responsible breeder.)
You'll probably be required to provide proof of home ownership or landlord's permission to have a dog. Depending on the dog's breed, the shelter, rescue, or breeder may require an inspection of your yard to make sure the fence is secure. In addition, the dog's current caregivers want to ensure that his personality and lifestyle are a good fit for yours. If you live in an apartment and you want to adopt a border collie, expect detailed questions—and regard it as a red flag if you don't get them. Yes, it's possible to have a high-energy dog in the city, but be prepared to show that you've thought through how you're going to keep him active, healthy and happy.