Oh man…it’s currently Halloween right now and you sent this 15 hours ago so I apologize but…maybe just alter a long sleeved shirt(chop off the left sleeve obviously) and then make a quick arm with a silver metallic sleeve or some craft foam? I’ve seen some pretty creative stuff, google some tutorials for his arm and see what you can find. I’ve seen people use metallic duct tape as well! There are a lot of cheap cosplay tricks but it’s such short notice! I hope you can figure something out, I’m sorry I wasn’t more help! If anyone has suggestions feel free to reply to this or reblog it and add your ideas!
I just made a really quick but fun Bucky costume this week out of an old jacket I had (removed left sleeve), some 99c gloves, and some metallic duct tape. I made the arm in two pieces so that it could come off easily and I could actually bend my elbow, and I wore a grey shirt underneath so the elbow joint wouldn’t show too badly. The gloves came as a set of one pair black and one pair grey, so I took one glove each and chopped the fingers off the black ones to the first knuckle. Then I used duct tape to make silver metallic fingers on the grey ones (leaving spaces so I could bend my fingers) and put the black glove over the grey glove.
I regrettably don’t have pictures of me wearing it because my camera was dead when I made it, but here’s the arm.

And the glove
I used some old cardstock I found to make the base for the duct tape. I found it gave it a much smoother appearance than just duct tape alone. It wasn’t even too terribly uncomfortable. I ate a meal in a restaurant without taking any of it off! I think it wasn’t bad considering I threw it together in a day. Hope this helps give you some ideas, Anon, and good luck!