When I first got my iPhone 3G back in 2008, I searched high and low for the dock adapter for my universal dock. I could not find it on Apple Singapore Online Store or in any authorized Apple Reseller Shops.
I only got it a year after (in 2009) when I went to Taiwan for my graduate trip and I saw it selling in an authorized Apple Reseller Shop.
When I bought the iPhone 4, I was thinking how to get the dock adapter for the iPhone 4.
One day I was browsing iPhone 4’s accessories on Apple Singapore Online Store and to my surprise they are selling the Apple iPhone 4 Universal Dock Adapter  for S$14 and of course, I immediately bought it.
Packaging Front View
iPhone 4 Dock Adapter
Labelled as 19 (iPhone 3G/3GS Adapter is 15)
iPhone 4 Adapter (Top), iPhone 3G/3GS Adapter (Bottom)