I?m just gonna cut and paste a post I made on another forum that should help you get started. But first, as I mentioned, 12 inch tall Showa figures (like the ?54 Godzilla you mention) are perfectly scaled with N scale model railroad accessories. And lucky for us, there are several major Japanese manufacturers that produce both modern and vintage (so, you know, you can have 1950?s and 60?s Japanese buildings, trains, cars, and people to go with you 1950?s and 60?s Japanese Monsters!). Some of the more popular brands are Kato and Tomytec.
It should be noted that N scale can be either 1:148 or 1:160 scale but the difference is negligible. But the important thing to note is that means that 1:144 scale candy toys from Japan also scale well and there are TONS of Japanese (and other nations) military vehicles produced at that scale by companies like F-toys. They are highly detailed and usually pretty cheap.
As for an 8inch G?84 (the Kaiyodo model kit?), that is 1:400 scale. You can find commercially produced cars and buildings and such at this scale but they tend to be pricey. But it?s also worth searching on Shapeways.com (a 3-D printing site). You can find tons of stuff made at whatever scale you need if you?re willing to paint it and maybe do some minor assembly and/or sanding.
As for finding this stuff, google and eBay are very useful. Below I?ve pasted my previous post from tohokingdom about this same topic. Hopefully the links still work!
This is just a handful of the things I found most interesting, but there?s a whole lot more out there at this and other websites,
JSDF vehicles
https://www.1999.co.jp/eng/10553698
https://www.1999.co.jp/eng/10346736
Fire trucks and firemen
https://www.1999.co.jp/eng/10472902
https://www.1999.co.jp/eng/10508922
Damaged buildings
https://www.1999.co.jp/eng/10402476
https://www.1999.co.jp/eng/10402474
https://www.1999.co.jp/eng/10513041
Showa era looking shops
https://www.1999.co.jp/eng/10472924
Showa era people
https://www.1999.co.jp/eng/10372926
My personal favorite: a film crew!
https://www.1999.co.jp/eng/10421237
I also stumbled upon this brand called Outland Models (mostly on eBay) that makes a variety of damaged buildings