are pretty Frequently Asked Questions in music newsgroups and forums, and I regularly receive one by private mail - some believe I know everything because I have contributed to this WIMA.
I often find the answers by browsing the OPAC's (online public access catalogs) of music libraries and sheet music stores. I find these catalogs more valuable than Web-search engines for this purpose. I have compiled a selection of the widest catalogs I know to help you answer your questions.
Some servers may accept one of these signs *, #, $, etc. as truncation character, e.g. sonat* will give all results with sonata, sonatas, sonatine, Sonaten, etc. I let you find the right character for each server.
I still have some work to do to find out which catalogs above are encompassed by these!
I thank all these servers for the help they provide. If you know of another useful server about the same topic, please let me know. I will try it out with my favorite obscure composers, and if I am impressed by the server's knowledge I will add it up to this list. And please let me know if either server no longer works!