Most travel adapters for power sockets are not only expensive and bulky but they have as well
connectivity problems. The contact is often not reliable and you have to shake the power cord until
it finally works for a few minutes.
In times of international shipping and ebay it's quite easy to buy power cords from another country.
Just search for "power cord" maybe in combination with the country name. Having a proper power plug from
the other country allows you to build a travel adapter.
The best solution is usually not go to the hardware store to buy a screw-on socket but to
cut off the end of a small extension cord or power bar.
Just get some heat shrink tubing and solder the cable ends together.
This allows you to build a very nice travel adapters not only for a single device but you can use
it to power multiple devices at the same time.
Here are some examples:
A travel adapter based on the 2 prong Euro-plug is useful for many countries even outside of mainland Europe:
Here are some other power adapters for Europe and China/Australia with proper ground pins:
