You should always keep your dog on a leash no matter how obedient they are. Accidents happen.
Glad you're looking for a harness for your other dog, but still get one for your obedient dog too!
They always have buddy belts on ebay. ChokeFree is another big one. There are tons of threads on harnesses tho, use the search
Heres link to puppia sizing:
http://puppiavest.com/puppia-vest-dog-harness/