Bob,
Lewmar sells the replacement lens and gasket. If you just want the lens, a plastics shop can cut you a new lens if you give them the old one as a pattern.
However, for the same amount of time investment, albeit more cost, you can replace the opening portlights with the new Lewmar model of opening portlights. The new model has the added advantage that it doesn't leak when it rains.

There is a thread, "leaking Lewmars" in the technical forum with more details if you are interested.

Dave