Polycarbonate or Lexan as it's more commonly called is the strongest, 300X stronger than glass & 30X stronger than acrylic, but also the most expensive.
Cast acrylic is the best grade of acrylic, it's the one to go for or Lexan if you're willing to pay for it.
I don't know if this is the cheapest place but it has a very good selection. They have cast acrylic up to 2" thick.
www.delviesplastics.com
The place has numerous thicknesses of cast sheet, tubes of differant sizes, UV colored sheet, mirrored..........ect. They also advertise that they will cut it for you, but I don't know what they charge for that.