I’ll keep it real with you - most people are not going to give you the best tips. Trying to get the best possible deals on Japanese cards is a competitive endeavor. But here’s a guide of the most important sites to help you get started: www.elitefourum.com/t/japanese-marketplace-list/21709/1
I will say that with the specific cards you’re looking for are incredibly rare. Let me give you some numbers:
Gold Trophy Pikachu No. 1 - 32 awarded (PSA pop 11)
Silver Trophy Pikachu No. 2 - 32 awarded (PSA pop 8)
Bronze Trophy Pikachu No. 3 - 32 awarded (PSA pop 10)
Super Secret Battle No. 1 - 9 awarded (PSA pop 7)
Super Secret Battle No. 2 - 9 awarded (PSA pop 4)
Super Secret Battle No. 3 - 9 awarded (PSA pop 7)
Illustrator - 39 awarded (PSA pop 19)
(This doesn’t include the extremely small handful of “file copies” that have emerged, but also keep in mind in mind that many of the awarded copies have never hit the market and never will.)
For the Pikachus, those numbers include all 3 tournaments in which they were awarded (the first 3 official tournaments ever).
In terms of recent sales - I am sure you already know about the SSB No. 1 that sold for $90k on Heritage Auctions and the Illustrator that sold on ZenMarket for about $250k. In addition to that, two 1998 Gold Trophy Pikachus recently sold for $88k and $85k on a little-known Japanese website called Mandarake.
You are seeking out the absolute holy grails of the hobby with these cards and if you want to pay what seems like a “fair” price for them right now you should be prepared for things to get very competitive. The only way to actually pry these cards out of the hands of an owner is to make a significantly bigger offer than whatever the last one sold for, and even then at any given time you’re not likely to get any bites.
I know all about how hard this is. I’ve waited the last couple of years to buy a Gold Trophy Pikachu only to lose out on it in the Mandarake auctions.