You won't find a Underarmor mannequin, I've been looking. And if you did some modification would be necessary. Limbs hacked, lipo in the butt! I've only seen one for sale and it was maybe 8-9 months ago on Ebay/craigslist. It sold for $250 on Ebay. I kinda wish I had bought it up. I do like Elm Studios approach but it is a bit time consuming. Tom Spina may have a suggest as he worked on two life size Terminator 2 statues. Bike and standing figure.
If you are going for a clean T2 figure I can get jackets. Basically I measured 2 real life jackets and use those measurements to have a jacket custom tailored for around $250.
But if you want the stage X damaged look, best jackets are the cheap thick leather jackets found on Ebay. Once in awhile a jacket with sidelaces pops up for under $100. They are nice to weather, rip up and have the right thickness/color that the Bates jacket have.
Lastly I make T2 grenade slings, about $175 each. They are very time consuming. I also have the grenades, depending on how many you need, and gloves $30 AND pants belt, also very time consuming, a steal at $75. I only have one belt at the moment and I'll probably ask $100 for the next belt. I don't think anyone else has ever offered the pants belt. The middle design on the belt is hand stitched and it has the correct, length, width and buckle (buckle is no longer made and costs $20 in itself).
I own a production made grenade sling and gloves so I got measurements from them. The gloves are nowhere to be found but I found some nearly identical. I've spent a considerable amount of time looking and researching Arnolds costume. I also took measurements from two T2 jackets as well as pants.
I'm also a PHD candidate in T2 props/costumes. :behave