I've found this at Southern Lumber in San Jose. It comes in 5x5 foot sheets 3mm (1/8th inch) for about $18. I have them cut the boards into 24 inch strips so that they fit in my car. I then cut the strips into 24x18 panels at TechShop. The price per panel is about $2.50
It also comes in 6mm sheets.
I've found that the glue in this plywood makes it harder to cut.
Settings for 3mm
45watt: speed 8, power 90, frequency 500. If the laser is really clean then speed can go up to 12.
Settings for 6mm