I have purchased many "for today only" bras from the Walmart -- it was that or attempt to dry out my sport bra on the hair dryer after a morning gym workout! Walmart is open long before most other places and close to my gym.

As for the stain: at the restaurant I worked at in college, they always told me to use soda water first with an oil stain. I have no idea if it's true. But it cannot hurt!

I second keeping spare shirts at the office -- I do this just because of my line of work -- spare EVERYTHING! But, we also have a washing machine on the premises.