You could try fastsigns in Bloomington - 2470 has been using them since 2008 and they remember us every year!
http://www.fastsigns.com/190-bloomington-mn
As for the polycarbonate breaking, we have only had that happen once. Last year we were lifting the robot into a car for an outreach event and the polycarbonate got sandwiched between the floor and the robot. Snapped right in half! Otherwise it hasn't been a problem.
We usually throw some industrial strength velcro on the back of the polycarbonate and velcro it to our robot. As long as you do not get any metal or wood shavings in the velcro fabric it should hold through all of your matches.
Feel free to pm me and I can try to find our contact for the Bloomington Fastsigns branch.