I lost the plug (duckhead) of my MagSafe 2 adapter, I know some people sell them on eBay, though I do not know if they are genuine/safe to use. Is it possible to buy them at Apple authorized repair sites?
It is possible that some Apple Authorized repair sites have them, but I would say it would be a long shot as most of those stores just sell 30-pin cables, lightning cables, and power bricks; nothing else. Your best bet is to go to your local apple store (if you have one nearby) or just order it from apples website or a more reputable place like Amazon. It is possible that the non genuine could work just fine but it could also break after a day or damage the power port.
Most Apple Authorized Service Providers (AASP) will likely have some that they could sell. Probably not new ones, but used ones or spares.
The folks at Twelve South make the Plug Bug adapter that would fit the bill. It costs a bit more than an exact replacement would probably be. I borrowed one for a month and used it every day. No worries about how safe it is to use it and it fast charges my iPad mini. There is also an international version if you travel or live outside of the USA.