Hi Karl
Yes we did a lot of research before buying and thought they were a good price, we looked at new ones at Travis Perkins and they were smaller and £30 each!
We got ours from a Garden Centre/Water Garden place and we paid £18 each for the reclaimed sleepers.
We used new ones for the top which were treated with eco friendly preservative they were £20 each and from a local guy who now also has the reclaimed ones, he is just outside of Beverley (East Yorks) and he didn't chargefor delivery.
I don't know where you live but if you ring around you might find them, as long as the ones which you use for the top are eco friendly, you should be ok.
Here is a link to the Water Garden place, they also delivered them for only £5 which I could not believe!
http://www.practicalfishkeeping.co.uk/pfk/pages/shopfinder.php?store=298&thiscounty=East+Yorkshire
Best wishes
D & G