It is not possible to search around the sketch code when viewing/editing it in the web IDE (spark.io/build).
Hitting the Cmd+F (or Ctrl+F in PCs) in my chrome browser doesn’t do anything, and if I open the find dialog manually via the “Edit -> Find -> Find…” toolbar - it only searches in the visible code. Meaning - if I have code that to see it I need to scroll down/up in the browser, the search will not find it until I scroll there, and when I will scroll there, the search will not find code that was visible before and is now hidden due to the scrolling I just did.