Here is one thematic dictionary of the type Mark Kinsler was asking
A dictionary of musical themes / by Harold Barlow and Sam Morgenstern;
introd. by John Erskine. London : E. Benn, 1978, c1975. xiii, 642 p.
(LC record no. 78-317090).
If you search in a good library union list under the subject heading
"Instrumental music -- Thematic catalogs," you will find several
general thematic dictionaries and also many dedicated to a particular
composer or time period.
Matthew Caulfield (Irondequoit, N.Y.)