Re Levy links:
You can't link directly to search results because the page is constructed on the fly using server-side scripts, and the search parameters are passed through a hidden mechanism (probably POST data). When you load the script page directly, no parameters are supplied.

That said, once you have the box and item numbers for a specific entry, you can construct a working URL by replacing the id value in the following example:
http://levysheetmusic.mse.jhu.edu/levy-cgi/condisp.cgi?id=060.066
(Clicking on the link above should take you directly to the entry for "Over the Garden Wall". While the Levy has multiple entries for this title, only one can be indicated with this technique.)

There may be a way to kick off a search in a similar manner, supplying all the search parameters in the URL, but I don't know what that is. You'd also have to encode all special characters in the parameters (particularly spaces in names and titles).

You can also copy links to a specific page of the sheet music, but those pages don't provide a back-link to the corresponding entry page, so I don't recommend it, even though it's easier.

Note that the Levy could change how they handle searches at any time, just by recoding a few base scripts; defining links in the manner I mention is unlikely to prove viable over the long run.