Where can I find them for ganaching in the UK?
Ebay and Amazon seem to be the only places
Is there a particular thickness I should purchase?
Fyne Acrylics should be able to help you: http://www.fyneacrylics.co.uk
Not sure where my first response to your query disappeared to! The boards don't have to be that thick as you are using them as a guide and before acrylics were popular, people used standard cake boards.
All the best & happy holidays! :)