Prices of Lister D's are 'unstable' wherever you look. I recently sold a basket case (all there though) for £10, which was £10 more than I paid for it (ooh er, I must be a dealer!) while good ones can fetch over £100. Perhaps somebody else on this NG has one lurking under the bench, but ebay is probably as good a place as any to shop if you are comfortable with buying sight unseen, also try local free ads papers and agricultural auctions, maybe even buy a copy of Stationary Engine Magazine, or at least stand in Smiths and look through the ads! Decide how much work you want to do and what you want to pay, there are usually enough around to be choosy, but make sure it is complete and that the mag works, having to source parts or rebuilding a mag can quickly push costs well beyond the value of a fully restored engine.
While you are at it get the lad a years subscription to SEM as well (sorry Kurt) - that should ensure he is hooked for life ;-)