Also, you can purchase service dog vests, patches, IDs allllllllll over the place -- there is no official mandate for a service dog to wear any identifier whatsoever, it's up to your preference.
Here are some links to vests etc:
Service Dog Vests | Service Dog Harnesses | Service Dog Supplies
Simple patch you could put on any current vest you have, $3.75:
Amazon.com: SERVICE DOG ADA Assistance Animal...
There are tons of online sites for this. Many of them also provide official looking "IDs"...but they are often so overpriced -- and they are NOT required by law, at all.