A great place to look for Japanese cards, especially older ones, are the big second hand Japanese marketplace websites. Mainly sites like Yahoo JP and Mercari. They can be a little daunting at first sight, but as you can imagine there’s a good amount to be found on them. The vast majority of items don’t ship internationally so you’ll need to use a Japanese middleman company. There’s tonnes out there (like Buyee & Zen Market) who buy the item for you for a small fee and once they receive it, ship it to you.
For navigating these marketplace sites and their search bars, @pokemontrader made this great post of relevant search terms. Also Bulbapedia has the Japanese for each card which you can copy paste.
In terms of 1st Ed Vs. Unlimited Japanese cards, like most things it comes down to print run and availability. For the majority of sets, the unlimited cards had a shorter print run and are therefore less available than the 1st ed. Sometimes slightly more so and in other cases they can be infinitely more hard to find, which often affects the price.
That said, a lot of people may not know about the cards unlimited print run or don’t care about editions and may prefer to have the 1st Ed version anyways; so prices can vary a fair bit.