Hello Michelle, welcome to the forum and good luck in your search for a new caravan.
A good place to start would be dealers forecourts, they often have trade in caravans which they offer for sale so you could try ringing a few in your area to let them know what your requirements are. Leave a contact number or email address and hopefully they will let you know when a suitable trade in within your budget is brought in.
You could also try Practical Caravans
Caravan Finder where you can enter your requirements to obtain details of caravans that might be suitable.
Preloved is a good website for online classified adverts, there is Ebay the auction site and of course Practical Caravan Magazine carries classified adverts for caravans and accessories.
Remember to inspect any caravan before you part with any money, ensure that the vendor has the right to sell the caravan via the CRiS scheme and in the case of private transactions Caveat Emptor, buyer beware so look for signs of damp and lack of regular servicing.
If you have any questions please don't hesitate to ask.