The AC input of most PSUs are 100 to 240VAC 50/60 hertz. Ideally you just need the Japanese power cord to go from the Japanese power plug to the PSU. A local Japanese electrical shop should carry these or a PC shop.
 
You can use adapters but you have to use an adapter rated for the current (amperage) you need to use. In some stores and airports you can buy power plug adapters but these are made for small electrical items like rechargeable razors, curling irons, etc.

All the above by RJohnson11 is correct.
 
Buying a correct AC power cable would be better than using an adapter, but you can use an adapter..The C13 connector end on the cable that goes into the C14 receptacle on the PSU that's commonly used is standard worldwide, the wall socket end may be different, of course..You can buy AC power cables for PSUs just about anywhere PC parts are sold.
 
The input voltage of the PSU is on the label..As in this image, the AC Input voltage can be anywhere from 100-240 volts..The input voltage will be auto-detected and auto-switched to using it..Some PSUs have a manual voltage selector switch on the back that you MUST be sure is in the correct position before plugging it in, or the magic smoke will appear.
 
Don't buy an el-cheapo cable where the wire size is smaller than the original one..Cables normally show the AWG wire size somewhere on them..The higher the AWG number, the smaller the wire size..AWG 16 is common.