For cutting with the BlackTooth, what is the best type of clear plastic/acrylic to use, and can you recommend a source?

The blackTooth will cut acrylic very easily. The type of acrylic (cast or extruded) is not important since the acrylic itself has 100% absorption with CO2 lasers. Any plastic retailer, even the home improvements stores will have sheets of acrylic. Avoid using plastics that will off-gas toxic and corrosive fumes. One example is PVC which is Polyvinyl Chloride. The burning of that material will exhaust chloride into the air and combine with the hydrogen in the air causing a chemical reaction yielding hydrochloric acid which is corrosive and highly toxic to humans. Always read the MSDS (Material Safety Data Sheet) for any material that you intend to burn with the CO2 laser.