I found my 380RS on yahoo Japan car auctions by accident, I didn't even know what a 380RS was, I was perplexed that one particular Nismo Z33 was massively more expensive than all the others on offer. Once I identified what it was, researched it and decided I wanted it, I asked Rick from RaceCarParts on here, to get the car checked out. He did, quoted me for the purchase & deregistration of the car & then selected an agent in the UK to handle importing & IVA'ing the car (If it's under 10 years old)
Basically, JM imports, HJA, New Era etc all have someone in Japan to source cars for them, but finding examples yourself is possible via auction sites.
If you're serious about it, talk to Rick, Jurgen or Oz about the kind of car you want. I'd recommend looking around yourself first to get an idea of what to expect - bear in mind the £ is laughably weak against the ¥ at the moment.