Let's start with an image of Evil-Lyn:
You've got a good start with navy blue swimsuit.
Hit up a Hobby Lobby, Michael's or other craft store and see what you can find. I'll bet you can fine just about everything you need there.
You should be able to fine some kind of faux-fur in multiple colors at a good fabric store. A couple years ago I did a Captain Jack Sparrow costume and found some faux-leather that worked quite well for boot cuffs. It came in tons of colors. I'd bet that could get you Lyn's boots. For the collar - hit up the pattern section. Simplicity, McCall or Butterick may have something that you can adapt.
For Armor components (leaves) and belt - go with lightweight wood and paint it. Or some sort of sculpy for the torso piece.
Arms and Wrist bands - hit up google or some costume shops.
The helmet will be the most difficult piece. You might try some medieval dress/arms website and look for a basic helm. Then start the customizing.
It's all very doable - and you've got plenty of time to turn out a great costume if you put the effort in. And by effort I mean $$$
Make sure you post pics - might even do a tutorial to help inspire people.