Welcome to the threads! I just recently completed my own Mara Jade.
The places to look for supplies would be for cowl and/or cape fabric I would look at any local fabric store, what I used for mine was the raw silk and then dyed it, but that is more on the pricey side.
For the bodysuit I don't think you want to skimp on that one so I would suggest either the shiny stuff everyone has or the matte stuff I purchased or you may get really lucky at finding something in a store. Just make sure that it is a 4-way stretch and not bathing suit material.
The leather for the belt and harness can be purchased from any leather working store, the place a lot of us use is online and has some brick and mortar places around the states called Tandy Leather.
For the harness back plate and cloak clasp they can be made easily enough with creating a mold and casting the pieces in Resin.
Boats I would look in clearance sections to see if there are any left or try online if you don't mind buying shoes there.
For the gun, it will depend on which style you prefer. You can either make your own with wood or plastic pieces, buy a Hasbro Hand Solo blaster and modify it, buy an air-soft and modify it or some people sell the assassin style one fully made.
For the shin guards, you can either make them yourself from scratch or buy a set of swat shin guards and modify them. Depending on what look you are going for depends on if they go all the way around your leg. For the Shannon shoot it is the modified swat gear and doesn't go all the way around, but for some of the comic book looks it does.
My suggestion if you haven't already is take a look at other people build threads because there are so many brilliant ideas out there that people have utilized along the way to completing there Mara's.