Along the shelter line, adult dogs often come from these shelters already housebroken and sometimes even with a little training. And there are shelters that specialize in pure bred dogs, but you'd be amazed at the pure breeds you find in general shelters and county pounds.
Things to keep in mind when choosing a dog:
1) Adult size (know how big the dog will get when full grown if you go with a puppy, so many dogs are turned into shelters becasue the owners didnt' find this out or ignored it, and when the dog became an adult, they were "too big")
2) Cost of owning a dog: food, toys, medical costs, licensing, etc. Dogs, even little ones, are not cheap pets (and with all the shots puppies need,and getting them fixed, they can cost more than adopting an adult dog)
3) Activity level of dog vs yours (Do you have time, space, energy to devote to a high energy breed? Or would a calm, quiet lap dog be more your thing?)
4) Who will be the primary care giver to the dog (who will feed, walk, pay for expenses, take through training, etc) and are they up to the task of doing this?
5) Do you live in the right type of place for a dog? (Ex: If you rent, does your landlord allow dogs or have any restrictions, if you own do you have a yard with a fence, any yard hazards like a pool or no fence, etc)
6) Are you okay with a dog that will be higher maintenence (require more grooming, etc) or do you prefer one that will be more simple?
7) Are you ready to commit to this dog for 10-18 years? Because owning a dog is a lifetime committment.