LOL! I was about to make a post for this myself, as sort of an analysis of MacReady's costume, primarily because my dad has talked me into making a MacCready costume for myself for Halloween (I figured though I'm trying to go for a close enough outfit on a limited budget and limited sources now that I've moved from Orlando).
So far, here's the information I've come across about some of the pieces of the Outfit:
MacReady's jacket is a NYC Style #674 G-1 Jacket made by Schott, NYC.
The Glacier glasses that MacReady wears in some of the scenes in the film are a well popular brand at the time (I can't remember the name for it and the site where I accessed the information is current down). Hard to say of the side guards on the sides of the glasses are black or brown in the film (maybe someone with the Blu-Ray can confirm which).
The hat is a Campaign Hat with the front, back and sides folded up. What brand and model is unknown. However, there is a company that has made a replica of the hat called "The R.J.", but its cost is between $250 and $425 (depending on what material you decide to go with and if you want it appeared to be aged or not).
Here is the website for that company and hat.
Still unknown about what type of green flightsuit, the boots, the blue hoodie as well as the grey shirt and white undershirt that MacReady wears right now. But so far, I've only got the boots myself. Will get back to you on the glasses (and if there's any knock-off brands for them available), after I check again later to see if the website where I found out about the glasses is up and running again.
Also, I found out something pretty odd. I noticed that MacReady has another type of shirt that's on with an odd white and brown pattern (it's clear in this picture, as well as when MacReady turns around after the Blair beatdown), but it doesn't show up in any other shot or scene. I don't know if its a goof or what, but it definitely seems odd seeing it.