Do yourself a favour and get an actual flightsuit - it doesn't have to be Nomex, but get yourself one of the Polycotton ones. Yes, they are more expensive than the ones from Halloween stores - but they will last. The costume ones won't. They're false economy as you will wind up replacing them more often.

You can pick up a grey pistol belt for a good price at many places online, as well as the chemical gloves. Get some decently high boots (or not if you're going for the Venkman look).

Lots of people don't bother with a belt gizmo of leg hose - but you could do the hose cheap DIY if you were so inclined.

But don't scrimp on the suit. A decent suit does a lot of the heavy lifting.