This is perhaps the most random store on Castro Street in Mountain View, but Tap Plastics carries a wide selection of different types of acrylic boards, of all colors, shapes, and sizes.
Tap plastics will also help you manufacture any sign or brochure kiosk or stand that you desire.
The staff is friendly and knowledgable; they can help you pick out the pieces of acrylic you need for your project or just make the project for you. We bought several sheets of transparent clear acrylic for a display we created, and the staff helped us pre-drill holes into the plastic so that it wouldn't crack after the drilling.
A great store for your sign and acrylic needs.