Why is it not possible to ask someone to act on your behalf?
There are a certain amount of Model Railway exhibitions each year which Brian (Shawplan) attends.
I can't see how hard is it to ask who is going to those exhibitions and email them a shopping list?
Even if you don't know anyone to ask, I have a feeling that certain moderators here will attend all the Major Shows Warley, Doncaster BRM, Scotland SCC. So, why not ask them and enquire at the next show and if what you want is not available I sure they could raise an order for you.
I just don't see what the difficulty is.