where to buy small pre-cut glass for overflow

[epicfish]

This would worth checking into. Does it bond to acrylic though (if that is the route i'll take instead of glass)?

GE Silicone for Windows and Doors WITHOUT the Bioseal material will work.

If you want to bond acrylic to acrylic, use Weld-On. If you use an acrylic overflow and want to bond it to glass, there's no great way to do it. However, I've used silicone and it held just fine.