I've seen the American set available on E-bay, but you have to be willing to bid high. I bid what we could afford on several sets and each time was outbid fairly quickly.
I did manage to find my American version of The Voyage of the Dawn Treader. Probably in 1988, I purchased the whole set in hardback after my original paperback set had pretty much disintegrated from frequent readings. I lost LWW when I lent it to a neighbour girl after I got married, and she apparently misplaced it. Whatever she did to it, I never got it back. She paid for it by raking and bagging leaves for us. I know that I have the other six books, but the only one I can find at the moment is VDT. The others are probably in one of the boxes of books that we have not yet unpacked after our move to this house over a year ago... we have nowhere to put any more books at the moment, so the boxes have remained stacked in the garage.