We regularly use Mattek glass bottomed dishes and well plates for live confocal imaging, they have a range of plate sizes and glass thicknesses that should suit https://www.mattek.com/products/multi-well-plates/
Otherwise ibidi do plastic bottomed plates that are suitable for optical imaging, though they only offer 24 and 96 well versions http://ibidi.com/xtproducts/en/ibidi-Labware/m-Plates
If you can choose, go for glass bottom plates, I can't think of any reason, why use plastic instead of glass (except the price). Try to look at the Cellvis, the quality is great and prices are quite reasonable: http://www.cellvis.com/.