I am looking to retrofit an auxiliary water heater to a right hand drive Mk3 Skoda Superb, is this possible and where would I go for the parts?

Are you going to do it, or give it to a service to make it? If the latter, I would look for n autorized Webasto dealer/center.

I am struggling to find anyone that is willing to do it to be honest

Have a read through of this thread. Includes adding aux heater to rhd, amongst many many other things.

Great thanks