For best quality imaging, you want to use a 170 μm (0.17 mm) coverslip. This is roughly equivalent to a #1.5 coverslip. There are also plate and dish options which use a plastic polymer instead of a coverslip. When choosing a polymer bottom pay attention to the thickness of the polymer, since anything thicker that a coverslip will reduce the working distance of the objective and can prevent you from using high NA objectives. Also standard PS tissue culture plasticware can be imaged through but you will not have good performance and is really only acceptable if you just need low resolution images. The following are sources of imaging dishes and plates we are aware of.
There are a large number of multi-well plates designed for imaging available that vary in cost, mostly determined by the material the bottom of the plate is made of. There are some important details to pay attention to when deciding on a plate to make sure that you will be able to capture the images needed for your experiments. In general you want a flat-bottom plate with black sides to help keep low background fluorescence and light scattering minimized.
Many companies can provide a sample of the different dishes and plates. If you are unsure of what you need this can be useful for a way to test performance of the plates on your samples.