Best is Wall Socket, if use extension might have interference so signal not as good. Can buy those that have slot to plug extension in one.
Basically u have a few options:
1) Mobo with built in WIFI (Mostly iTX form factor)
2) PCIe Wifi Card
3) USB Wifi Adapter
Id suggest trying 1 of these 3 options first, then if signal is not good enough, try using those Homeplug/Powerline Adapter/Wifi Extender/Wifi Repeater etc.
Personally, I'd go for a PCIe wifi card if you cant get a wired connection
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