The first place to look for icons is on your computer: it generally contains several tens of “hidden” icons. GConvert lets you scan your folders in order to get all of the icons they contain: it searches for icons in .EXE, .DLL, .ICL, .RSRC, .ICNS, .ICO, .OCX, .SCR, .ZIP… and displays results in a list:

Search results can be exported to icon libraries (ICL) and/or opened in GConvert.