Richard,
If it's a classic caravan, or one with a very active owners club, or a caravan with a specialist following - then I'd go with that caravan forum or owners web-site.
If it's a generic white shed of a caravan - then maybe the owners club is worth a go, or somewhere like preloved.co.uk - it's free and has a good following.
Caravanfinder.co.uk might be worth considering - about