Where to buy replacement water filter plug 2008 23fb

[bonginator]

When we bought our used 08 23fb special edition, it came with the water filter already screwed in under the sink. During the winter last year, the filter expanded and broke during the freeze.

We replaced it this summer and now that it is time to winterize again, I want to remove it and put a screw in cap in place of the filter itself. Our airstream did not come with a cap.

What is the best place to buy a replacement cap?

Thanks.