Try that well known on-line auction site. We got two offcuts for a great price which do the job without taking up too much space. Very comfy and have cured my wife's recurring 'caravan back'!
I have just ordered one for my better half for when we are in the Van. I am getting it from Raskelf, I know it is expensive as she is having the gold 2" one for her back. If I can get it right the website is.
I find Raskelf well over priced in comparison to a lot of bedding shops and also eBay. We have two singles and in all honesty, I coudl not recommend them as they hardly make any difference. Wewent and bouht a proper topper at a lower price and it is far more comfortable.