You could try the different sites on line, just be careful of those selling junk that won't be accepted by the 501st.
You could also ask some of the people on here that have that costume, maybe they can tell you where they got theirs from or at least more info to get you started.
If you think that just posting a thread saying you want to buy this costume is going to lead to you getting the costume, majority of the time, not.
You have to be pro active, Search online, ask people questions, search out and find the answers you want.
Research can be brain numbing, we have all been there. When I made my starkiller, I got zero help from my Garrison or on here really. But you know what, if you really want to have this costume, you will find a way. If you don't invest the time, energy, and money it takes, you wont get very far.
I like to think of our Costumes as the exotic animals of Star Wars. Some costumes in this Detachment can run as high as $8,000, if you can even find someone to buy the pieces off of. If people are looking for a new costume, but don't have the money, time , and energy to invest, I would suggest going over to the KC clan and making a Jawa. There is a woman over there "Pixeldust" who does great work and you can get a Jawa costume done for around $350, for the basic approvable Jawa, all you need then is 1 Bandolier and some rubber boots and some glue.