I've been collecting these for a while, and I actually have duplicates of some, which I was planning on selling sometime soon.
(I've bought a bunch of "big lots" for the sake of one or two guides I need, leaving me with duplicates).
PC Engine-related magazines too.
I can't put it all up here right now, but I'll try to put something up in a few weeks.
Since you're going to Japan, try Akihabara (in Tokyo) first. Lots of game shops - though older things are getting harder to find.
Check out Super Potato first - best chance of finding stuff, but maybe not the best price you'll find if you spend all day:
https://maps.google.com/?ll=35.699347,139.771623&spn=0.002082,0.005928&t=m&z=18&layer=c&cbll=35.699347,139.770596&panoid=-qBnxIvD6DowNCKaL7pBxA&cbp=11,101.23,,0,-16.96If you're going to Osaka, check out Nipponbashi - I've seen some guidebooks in the game shops there too.
Book Off has been hit-and-miss for me, depending on the area/region; generally PC Engine stuff is too old/niche for most locations, but you might luck out.
I was thinking of going to Jimboucho in Tokyo because I hear they have lots of specialty bookstores... I'll let you know if I find more information.
But I think most of these guidebooks are on Yahoo Japan Auctions.
There are a few people and companies who can help you to buy stuff from there, but don't expect the final price to look anything like the apparent auction price, once all the shipping and service fees are included. It could still end up cheaper than eBay though.
Example search:
http://auctions.search.yahoo.co.jp/search?p=PC%E3%82%A8%E3%83%B3%E3%82%B8%E3%83%B3&auccat=21600&aq=-1&oq=&ei=UTF-8&slider=0&tab_ex=commerce