I would recommend getting the shirt and Cargo pants at a surplus store but you can get them online. Links below.
1. Give yourselves plenty of lead time
If you give yourself plenty of time up front you can streamline your shoot to the most minimal budget.
For my Enter the Freeman short film we also did a partial dress rehearsal the two days prior. We did not want to be slowed too much by blocking and I was also able to iron out issues with camera and lighting.
Ok this was my process. I don’t necessarily believe that this was the bast way to do it. It changed as the project progressed.
I read online a lot about how people had made cardboard versions of the HEV suit that they had then applied a material like bondo or fiberglass to it and then painted it. This was my first approach as I had had some small amount of experience using fiberglass and resin.