Try http://anaspecialeffects.8m.com/breakawaylist2.htm  (I've heard they do custom window panes)  or make candy glass yourself on a cookie sheet.  Here are one set of directions that I've found  http://www.likeastory.com/boards/cgi/noncgi/Forum18/HTML/000294.html YouTube has a video of two kids doing this http://www.youtube.com/watch?v=dZBuHglAGhY   This video is funny & cute: its kid filmed complete with little mistakes and 'Oh yeah and you also...'.  Or google it yourself with candy glass or sugar glass.

What show are you doing? Sometimes the break can be faked with simply a sound cue. When we did "Noises Off" there's a scene where Selsden enters by breaking a pane of glass and climbing through the window. We simply used a solid piece of plexiglass and he would push out the entire pane. (It was held in place by bent staples. No need to replace the glass; simply re-used the same pane over and over.

I would be concerned with shrapnel hitting the audience if the area is that close- New Orleans is hot, and windows are normally open- I've always seen that scene as the radio getting thrown offstage, and a crash box providing the sound of the radio crash landing wherever.