Hi TripleT and welcome to Triumphowners.net.
If you have a local Triumph dealer in your area you might try their first for these parts. But the best source I've found for non-current model year Triumphs is Merritt Marine in Hillsborough, NC. Dick Cabble with Merritt helps us out with our Tech Talk forum and they have a website with a ship store where you can order RULE:
https://www.merrittmarine.com/store/...uct_detail&p=8
You can also use their Parts online form to request a quote on the box cushion and leaning post, etc.
http://merrittmarine.com/parts.htm
Hope this helps. Again, welcome aboard.
Randy