You can get it at tractor supply or any feed store. It is the injectible kind. If you cal your state department of agriculture they can come out and test, depending where you are you may or may not have to pay, it shouldn't be too much.
At least if you keep them home, treat them and have them tested you won't have a guilty conscience about bringing them to a show and possibly infecting other birds. If they are clear and it is just a minor thing then awesome but you don't want to be "That" person :/ this year all chicken swaps were frozen for MONTHS because people were knowingly bringing sick birds and unsuspecting people were buying them. Before anyone knew if tons of flocks here in NH were infected with MG