Genuine original early Skyline motorsport wall posters are rarely inexpensive. To get an idea what's out there check out Yahoo Japan Auctions.
Type "Hakosuka" into Google Translate, translate from English to Japanese ( ハコスカ )
Copy the result and paste it into the Yahoo JP Auction search window. You should get thousands of articles on auction.
Here's a bit of a head start for you that only looks for paperwork, books etc.http://auctions.search.yahoo.co.jp/sear ... e&slider=0
Copy and paste it all into your browser search window and hit enter.
I use Import Monster because they are in Australia, but any similar agent or connected good friend will be able to get what you want sent to you.