I looked through all of the Hobie parts list I that could locate and couldn't find that drain insert. Because it's integrated into the hull, it might be considered part of the hull itself and not a service part.
That said, contact your Hobie dealer, they may have more information. You may need to contact Hobie themselves.
There may be an aftermarket company that sells such a part. However, be careful. Mounting a drain onto a polyethylene hull is not that easy. Polyethylene is a
low surface energy material and it may require industrial methods to get it to adhere properly. You have to be very careful with below the waterline modifications.