Hi Stephen
The gasket is a generic product. 1/2” round neoprene gasket. In Canada, TTL is a national supplier with branches across the country. Otherwise, should be avail anywhere. I paid about 1.25 Can per foot for this compared to $7 US from Atkins Hoyle.
Hammerhead is good for proprietary parts for many of the hatches and ports as well, at prices far less than Atkins Hoyle.
You can email me for more specific info if you like.
Greg Silver
Misty Cat 26C #121
St. Peter’s, Nova Scotia