That seems like it would be a really comfortable dress to wear. You know how you can pick different characteristis for the dress she wears? Soft, shiney, et cetera? I'd almost always
pick "soft" as one of my qualities-- in real life, I'd probably make mine of of very soft, cottony red material or something. I imagine that you could customize your own Aeris dress to fit the description you'd like most.
As for finding items that would be great for cosplay -- go for it! Making your entire costume yourself gives you bragging rights, but if you find something that would already be perfect for an outfit as it is (and is a lot cheaper than buying the material for it!), it would probably save a lot of time.
A perk about creating your own is that if you need more material for what you're creating, you can go to the fabric store and get some more of that same kind. If you buy something already made, finding matching material might be a little difficult.
Good luck with your dress! I can't wait to see it!