Well, in my opinion, one of the most imporant shots for chickens in for Marek's Disease. This vaccination is given when chicks are one to two days old, subcutaneously in the back of the neck. Marek's Disease is very serious, so it is a good idea to vaccinate for it. I myself raise/sell birds, and I vaccinate my chickens for Marek's Disease.
Other vaccines can be purchased. Many people who show or breed poultry vaccinate their birds for Infectious Laryngotracheitis, which is a acute, dangerous respiratory disease. Others vaccinate for Fowl Pox, especially if this disease is prevalent in their area.
You can get other vaccines, like Newcastle's and Bronchitis. Both of these vaccines are good to give, but not as important as the Marek's Vaccine. Newcastle and Bronchitis vaccines are given either in the water, or in they eye.
I do not believe that you can get vaccines at
Tractor Supply Stores. You'd have better luck looking online. Twin City Poultry Supplies (
http://www.twincitypoultrysupplies.com/store/) is a supplier of vaccines.