Elephant trunks = collapsible deck drains?
Might be easier to make them yourself. Just a rectangle of hypalon glued into a tube, as far as I'm aware.
For the valves, it depends on who the manufacturer is.
I think IBS (Inflatable Boat Specialists) used to source parts for Avon, but I don't see that on their site now. You might try searching for the valve manufacturer on various river raft manufacturers sites (there's more of them than SIB sites.) Here's a starting point: