Slowly but surely the cold water to my tub stopped flowing. I've been through my basement looking for shut off valves. No luck. Help.

Is this the only affected fixture, if so its likely a clogged inlet on your mixing valve. Do you know how to service a mixing valve?

This is a mixing valve and not just a single hot and cold valve? If this is a mixing valve shut the water off to the valve and open it up. Remove the cartridge and check and clean the inlet ports. Before you reassemble, turn the water back on for a few seconds to flush out the supplies. Reassemble and test. Good luck, Tom