It really depends what type of caravan you are looking for. If you’re looking to buy a mobile caravan, some places you could try include:
People often use sites like gumtree to sell their old caravans, cars etc. You may be able to get a steal if you find a seller that’s looking for a quick sale
If you’d rather purchase a static caravan then you could try speaking to people on sites that you might be interested in. When I was searching for holiday homes for sale in Kent, I visited a lot of static caravan sites and they showed me some options. You can also try searching online for Caravan dealers or holiday homes for sale as some sites will advertise the plots that they have for sale on their websites. This is how I found the caravan dealers in Kent which I bought my caravan from.