What kind of puppy should I get?

I am between the ages of 12 and 15 and I want to get a puppy for my birthday. I want a small dog that will stay under 30 lbs, and I want it to be sort of laid back, but one that would want to walk/jog with me. I looked at Cavalier King Charles Spaniels but I can't find any for under like 700. does anyone know of a place to get them for cheap, or have any ideas of other puppies that would fit my requirements? Thanks in advance :) !