Where to buy pre-builds gaming PC’s in Tokyo

Hey guys, I'm an avid builder of gaming PC's but this one will be for my cousin so I wish to get him a pre built (so I don't have to support it too much if it should break in the future).

Where do you guys go to buy your pre-builds? I assume that yodobashi and big camera will charge too much for mediocre gaming pcs's so I'm looking for a company that only builds PCs.

I've found ARK PC, Dospara and Mouse Computer but I'm not sure if they are any good. Dispara seems to have the best value for money though.

Thanks in advance.