Here at TSC we measure our seaglass by how much we can fit in our hand. We maybe ought to review that in future.
The seaglass we collect comes from South Lanarkshire, Dumfries and Galloway, Fife, and many of the major lowland rivers. We go hunting rocks but we also remove litter, metal scraps, fishing lines, and other bits and bobs from the water. Seaglass is technically a pollutant, but it's a pretty pollutant that we can never bring ourselves to throw away. 
This handful of seaglass from Scotland has come mainly from Ayrshire, the River Clyde, and just south of Edinburgh.