Pony Club websites are generally very good for this sort of thing, word of mouth as well. Websites like Horse Mart aren't bad, let friends know you're looking, and they'll tell people they know etc
Try local feed merchants, they often have a board up with ads for horses for sale, share, loan etc. If they do, you could post your own details, that's how I found the person who helps me keep my horses fit.
Found mine on preloved. Took about 6 months to find her, had lots of people who were time wasters. So prepare yourself and don't build your hopes up. Good luck.
Hi it is hard to find a share/loan and I was at the stage of saving to buy instead when my current full loan boy came up through word of mouth. Prior to that I placed an advert online and in my local paper offering my help with chores in exchange for riding out and had several responses. Local livery yards often have a notice board in their tea room, you could always pop a notice up? Sometimes people arent asking for help until they see the right person looking to give it Good luck with your search.