I have a website online that I love to buy my leads and collars from (although collars I can quite often find at the show from suppliers, leads are much harder as I want more length and strength than is offered by suppliers at all breed shows).
Leather Leashes, Leather Collars, Choke Chains - Leather Leash Store
I have several snake choke chains I use at allbreed shows, fur saver chokes that are more popular at specialties, nylon choke chains I like for puppies. And I am looking at getting some leather martingales to use for puppies/adults too. I leads from here I love, they are the premium show leads and I have them in 8, 10, and 12 feet.
I personally prefer wearing pants, but that is mostly due, shall we say, to my size. Most of the handlers that show for me wear skirts, short - skirt overlay, a few wear pants. It really depends on what you are comfortable in and I would recommend you have some choice for weather as well. What you wear at an summer show is different than what you would want in the fall or even winter show. Just make sure that whatever you choose shows off the dog! You don't want the dogs topline to blend into what you are wearing. For instance, for my bi-coloured pup I wear white pants with a red top, very flattering for her black. GSDs normally being darker with the saddle and such would need a lighter colour to stand against, but again you don't want that colour to match the red/tan points of your dog as well. Basically make sure you don't match your dog, but contrast nicely (accent) your dog.
p.s. I will come back to training your dog to drive out.