If you want parts, rather than just a score, include “and parts” in your search. For example:
Brahms string quartets parts
Usually string quintets are for 2 violins, viola, and cello. But string quintets can include either 2 cellos or 2 violas. If you know that you want repertoire for the 2 violins/2 violas/cello combination, you search:
quintets strings “violas (2)” parts
or
“String quintets” “violas (2)” parts
This information is stored in the subject field of the item's record in the catalog, and you can in fact search just that field instead of doing a keyword search to be more precise. This looks like:

In a record for an item it looks like this (the display says "Topics" instead of "Subject".

You can use any of the above tricks both in the UIUC catalog; in the iShare catalog; and in WorldCat.