Try joining the eribaforum.co.uk - occasionally older Pucks come up for sale on there. Eriba owners tend to be very loyal to the marque so I have some doubts you'd get someone willing to swap but you never know.
The Puck is definitely a good way to go, the construction is so much better quality and they tow brilliantly.
Mine's a '92 Elddis Wisp 350/2, not as retro as these but a bargain at £400 I reckon, fridge, cooker, hot water, shower, toilet, gas heater, comfy double bed and feels like the Hilton after years in a tent! Been to Cornwall and back twice with no problems and it's really easy to tow.