Where to find valves like this?

There are other ways of removing the water, depending on low point drains is asking for trouble, but in any case try http://catalog.winnebagoind.com/menu/Parts.htm
Winnebago is very good about stocking obsolete parts for their rigs.

Thanks for the link - I think I have identified the part number as 052960-01-000 and I have contacted Winnebago.

But how else can you remove the water without using the low point drains?

Steve