We have a couple of hooks similar to that on our shower door that was left by the previous owner. They are metal like the ones you show, but the hooks are slightly different.
We also have a over-the-door towel bar that we hang on the inside of the bathroom entry door that we bought on sale at Camping World (ours is chrome though):
Over Door 3 Bar Towel Rack - Bronze - Interdesign 69111 - Racks & Holders - Camping World
Sometimes, Camping World's on-sale items have good pricing.