Roger I am not certain how you plan on using the pumice and so I will give you a choice of sources I know.
Some beaches have an abundace of pumice that washes up on the shore.
Blocks of Pumice stone can be purchased at some cleaning supply stores. They are used to clean porcelain fixtures. They vary in courseness and can scratch the surface of the silver easily. This might be useful as a first step in finishing silver.
Small pumice stones can be acquired for annealing pans from jewelers supply stores. they tend to be more consistant in coaseness and might work for small items.
Powdered pumice is available at some paint stores and are graded for finess. I have used this with water to finish some pieces. Pumice is the abrasive ingredient in cleansers such as Bon Ami, Comet, Ajax, etc. Most of these contain other cleaning ingredients as well.
You can also purchase fine pumice wheels for your flex shaft that can be used quite well for finishing small areas.