Vernon Library Supply has clip on shelf labels that are great for labeling:
books (by category, reading level, author, etc)
Lab equipment shelves
The great thing about them is that they are easily movable, so adding and subtracting items to your shelves won’t throw off your whole system.
If you prefer the traditional look, House of Antique Hardware has a good selection too. Especially if your shelves are out in the open and not in a closet, behind doors.
Can you tell I’m getting excited about our upcoming move?