Depends on your skill level, as well as dollar.
As a seamstress, I vote make it from scratch. It's incredibly cheaper. I have three premade Phoenix suits I got before I started sewing, and I've altered them since. Already in the works to remake them entirely, but Phoenix is such a popular thing right now, I prefer making suits that aren't
If you're not comfortable with making it completely yourself, buy a premade, and alter it. If you have a serger, make sure you use this as they're best for spandex.
I know Halloween is already this week, but any progress pics? Did you end up doing this? Good luck!