I am one of our Garrison Membership Liaisons and happy to help as you begin your journey! You've come to the right place. Be forewarned that a Vader costume that would be approveable for membership is going to expensive and time consuming.
Some folks go the route Deaftrooper mentioned, and do an "easy" costume to get membership and get involved, but that may not be the right path for everyone. I say "easy" in quotes because none of this stuff is usually off the rack, ready to wear. Research, research, research, and ask a LOT of questions before you spend your hard earned Imperial Credits!
My best advice is to join the forums over at the Sith Lord Detachment. They are the home of all things Vader, and the keepers of our costume reference libraries pertaining to all Sith related characters. The CRL, as we call it, is the minimum criteria your costume has to meet for membership.
You can find the Sith Lord Detachment here:
And keep in mind there are a few versions of the Vader costume eligible for membership. You'll need to decide which version you would like to build. The requirements for each can be found here:
We have several GLG Members with Vader costumes who I'm sure will be happy to help and again, you will find an invaluable wealth of information on the Vader costume over at the SLD forums.
Some of the other costumes you mentioned can be researched at Jolly Roger Squadron (TIE Pilots and Reserve Pilots)
Imperial Officer Corps (Officers and Crewmen)
And Imperial Gunnery Corps
If you have any questions in the meantime, please do not hesitate to contact me via PM here or via email at email@example.com
Good luck with your project!
John -stasz- Studniarz
Garrison Membership Liaison