I purchased my acrylic at Canal Plastic. If you are in the NYC area then you should know about Canal Plastic, its an excellent resource for almost any kind of plastic you want and in all kinds of shapes or custom cut.
The acrylic I used is 1/4" clear plexi with the long edges polished. All the sides that are going to have LEDs should be polished. You can polish this yourself but having a professional do it will give you a much better result. Most establishments that will cut the plexi for you will also have polishing available for a minimal cost. It comes with a protective paper glued to both sides of it, I wouldn't remove this paper until you are ready to test your LEDs.
While you can get your interface working with just the acrylic, if you want it to work well then you need to have a projection surface and a sheet of very thin clear rubber.
The projection surface can be a real projection screen or almost any kind of paper. For Schproket I am using sheet vellum cut to the size of my acrylic. This is simply for the projection to catch on something. If you don't have it the projection will go right through the plexi and you won't see the screen you are touching. The only restriction on material is that your surface can not stick to the silicon sheet, try testing a few materials to see what works best. Remember that this is also probably what people will be touching; so a material that is durable or cost effective replaceable is your best bet.
The rubber needs to be very thin and transparent or semi-transparent. The common consensus is on silicon rubber for this. Many people are using Sorta Clear 40 to produce this sheet of rubber. I am not because I have never had any good experience with SmoothOn products. For Schproket I ordered Near Clear from MicroMark, which I find to make far superior products.
This is kind of tricky because you need to make the sheet of rubber. Why? If you set up your entire interface without the rubber it will work, but you will have to press extremely hard on the acrylic to get a decent amount of light to leave the glass. This is about FTIR (frustrated total internal reflection). I am not the expert on this but essentially the light from your LEDs is bouncing around inside the plexiglas. When you press your finger on the top of the acrylic your finger is interrupting the flow of light within it. That forces the light to leave the acrylic directly away from your finger. With just acrylic this is difficult, adding the clear rubber makes it easier. The light is now bouncing around the glass and the clear rubber. When you press your finger on the rubber, which depresses due to its spongey nature, your finger is making a very clear depression in the flow of light. This makes the light leave directly away from your finger. Basically its easier to push rubber than it is plexiglas.
So...we make a sheet of rubber because I have yet to find a place that makes this.
You should make a molding frame around your sheet of acrylic. This needs to be tight against the sides of the acrylic and 1-2mm taller. You are going to pour the silicon mixture directly onto your acrylic.
Follow the instructions for the silicon rubber you have purchased. Be sure to pay attention to the mixing instructions, you want to avoid bubbles forming. Its ok to have some bubbles, its almost unavoidable to have a few.
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