For plexi or acrylic I use Weld-On #16 from a plastics supplies place in town. For glass I use the smaller hobby size tubes of 'aquarium safe' silicone sold at Home Hardware. Heres a picture of a large size tube, so just look for the smaller ones that look like tooth paste toobs and have the same colors and text.
http://i29.photobucket.com/albums/c2...r/P1040709.jpg