Ok, here is the scoop from over here:
Fine Molds:
Does a lot of AM parts for non-sci-fi stuff and similar their SW kits are not the ONLY things they produce. They started with some Porco Rosso(Gibhli Movie) planes and than moved into the SW range.
Bandai, Imai(now gone), Arii, Aoshima, Takara:
Make cheap and fairly low-quality plastic models. Nuff said, as their main-business is toys not plastic models.
Bandai used to make some great Plastic models, but traded it now in for the snap-together range of kits. Their big bonus is that they own a LOT of licences and thus can produce plenty anyway they want.
Wave:
Is one of the bigger Resin Kit producers in japan their FSS kits are normally in 1/144 scale and their they are second only to Kaiyodo(no longer making theirs).
Some of their new Plastic FSS offerings are good but do not stand to to the resin versions of them.
Volks:
Got plenty of money? You will need it they are the top resin kit manufacturer in japan, know it and charge accordingly.
Most of their Kits are in 1/100 scale and are very good.
Some of their larger items come with a repayment plan, like for the 1/8 scale Scopedog (1.200USD) or the Twin Tower FSS Kit (2.000USD) which stands 1.68 meters tall.
Yellow Submarine:
Also has some very decent Resin kits on offer.
Modelkasten, Silicone Tribe and others:
Offer Resin upgrade and pure resin kits of SF3D Material, very good and pricey.
Argonauts:
Owned by Aoshima at times does re-issue some kits, like the 1/4 T2 Endoskeleton they recently did.
Halycon:
Gone.
There are also a LOT of small kit makers that hardly ever advertise and usually only sell on a very local scale.
I got a 1/100 Macross Fighter Pod from one of those and the quality is as good as the big companies.
In order to find the really good and high-quality kits in japan in the Sci-Fi range you will need to leave the plastic market behind as they are normally done by Toy-makers until Hasegawa and some other companies stepped in.
Looking on the Web it looks like the Sci-Fi kits worldwide have all gone over to the Garage/Resin kit scene, meaning that it will be available but cost you some money, time and effort to get them.