Suggestion: There are various books listing circuit for devices that may be built by hobbyists. Some books were published by TAB Books, Blue Ridge Summit, PA 17214, USA, others appeared in books similar to:
John Markus, "Electronic Circuits Manual," McGraw-Hill Book Co., 1971
However, as the previous posting suggests this approach is more time consuming and more expensive than to buy the adapters or power supplies commercially off the shelf.
Also, there is a potential hazard since the sold product usually comply with industry standards, e.g. Underwriters Laboratories (UL) so that they are considered safe. If you produce some power supply and someone else gets hurt, you could be held liable for the injury or electrocution. Therefore, you have to exercise the utmost caution.