Welcome to the forum and best of luck with your search. I'd try the usual places, e-bay, gumtree, possibly facebook marketplace.

Colin

They do come up occasionally, but have never sold in the large numbers that the 530 and 540 do. They are a really nice layout with the large fridge and combined space/water heater, but you need to read the specification carefully when you find one. hen on hook up the 12v fridge ran from caravan transformer along with standard gas heater always seemed strange to me. You need to find one with the factory optional electric element as I believe. it cannot be retro fitted.
For a main dealer then Lowdhams have what looks like, from the photos, a decent spec van at a fair price. Could be worth a call to ask about the heater and enquire about a free mover to be fitted. https://www.lowdhams.com/used-eriba-troll-535-caravan-338551

Colin

Edit - The other thing to consider is they only became available to order in 2017/18, so not that many will be available at a nice price.