If you want to use any of these, 1) Navigate in the Finder to /System/Library/CoreServices/ 2) Open "CoreTypes.bundle" by right-clicking it and selecting "Sow Package Contents" from the centext menu, then open the "Contents" folder, then "Resources" 3) There they are! Find a suitable icon (preview either with "Get Info" or by double-clicking, which will open them in Preview) 4) In a second window, navigate to the fol…
You can also press Shift + Command + G and paste /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/ to go there directly. Unfortunately, even if you set a custom icon for a folder, it'll still show as default icon in the sidebar. I wanted to give my ~/dev sidebar entry the developer icon, but sadly it seems to be hardcoded ~/Developer indeed.
Yes, this is a favorite petty grievance of mine: this used to work until some years ago and it was very helpful for quickly navigating to key folders, like those used for your current project, as you hadn't to actually read the sidebar items. For another few OS iterations this could be fixed by add-ons, but then this also wasn't an option anymore. (Apple really invested into making this not an option. I guess, this coincided with Jonathan Ive's takeover of interface design, when clean looks according to design templates became more important than usability.)