I'm trying to find a solo arrangement (i.e. without the orchestra) of the adagio from the Concierto D'Aranjuez. Anyone know where I might find one?

Like
this one by Kaori Muraji (must be a pretty old video)?

It has a piano accompaniment, I don't know of any completely solo arrangements.

The piano/guitar arrangement usually sold in Japan is a Spanish import published by Ediciones Joaquin Rodrigo (I think there may be another Spanish publisher putting it out, but maybe it's the same edition re-sold).

I see that musicroom stock it, but it would be worth phoning or e-mailing, because they seem to have a raft of different editions - guitar and piano parts published separately, excerpts from the concerto published separately, etc. (And possibly the excerpts are for unaccompanied guitar???)

Have you tried Guitarnotes in Nottingham?

Seconded! Forgot all about Guitarnotes, but they have the widest range of guitar music of any online source that I know, and have been great about sending music to Japan (the ABRSM syllabus includes some items which are not well known here).