Styrofoam is horrible for the environment. Maybe consider a different kind of cup such as paper.
That’s an especially odd request coming out of Eugene, Oregon. If anyone cares about the environment, it’s Eugene, Oregon!
If there is a CostCo nearby, find someone with a card. Otherwise your best bets are office supply stores. (And yes, Wal-Mart or Target.) It would be nice if you could try instead insulated paper cups. They feel a bit like thin Styrofoam but are infinitely better for the environment. Plus they are more compact in storage—they store more like paper or plastic cups than big, bulky Styrofoam.