For the Marines, height standards for male applicants range from 58 to 78 inches. Height standards for female applicants range from 58 to 72 inches. The services have both height and weights standards. If you fail the height / weight charts, but pass the body fat standards you are still eligible for service.
Most dogs require 1-2 years to become a service dog and be fully trained in public access skills. The training cost of working with a professional trainer at $150 an hour would be about $7,200 a year.
Now the short answer to the big two questions. 1. If you don't already know where to get a vest, then odds are you shouldn't be getting one anyway. There's a lot more to a service dog than the outfit it wears. It typically takes 18-24 months to fully train a service dog.