Once you have the size and shape try :Grainger Industrial Supply
On my Atkins Hoyle rectangular portlites I used their 3/8" round at approx $16.00(us) for 25 feet length. The 25' did 10 portlites. Have had it installed for 1 1/2 years and no leaks
. Yeah It sure beats the $40 per portlite that Atkins Hoyle wanted at the Miami Boat Show
. Grainger offers round, square, rectangular, oval and other types and sizes. You can order on line and pick up at a local store or have shipped to you.