Armpit pads instead of undershirts?

Hey guys. So I was recently invited to the End of Harvest Party at work. I work at a winery and this is sort of our version of the classic Christmas work party. This year it's is themed as "Midnight In New York". I have no idea what that means. It's definitely a dress to impress type event. I pretty much don't have any formal attire save for a couple of dress shirts and one grey pants/jacket combo. I expect to be buying everything and there for would like to keep costs down. Any thoughts? Much appreciated.