I had to face this exact problem at the end of November when my 31 year old mare lay down and wouldn't get up again.

The vet came and put her to sleep and was able to advise me on who to call. In my case, it was the local hunt who came with an individual trailer but as she was injected rather than shot, she wasn't fed to hounds, but they were still happy to help and very kind and sympathetic.
Good on you for thinking ahead and being prepared. Horrible to have to deal with that situation, but even horribler to have to watch them suffer.