You can use regular hard wall 1" marine water hose. Just cut it to length. We don't use any specific molded hoses from the thermostat housing to the manifolds.

Or use something like this 1" to 1" adapter to extend your existing hose. Just clamp it inline with your existing hose and run an extension of 1" water hose to the new 90 degree fitting in the water tree if you end up going that route.
https://cimg0.ibsrv.net/gimg/www.off...e847c50a1b.jpg