Carfromjapan.com has the best search features I've found, once you know what you're looking for https://www.goo-net-exchange.com/ is also nice because they translate the car condition sheets.

Parts availability depends on the car. For the Rasheen for example most of the engine parts can be found at any parts store for the 1500 and 2000 cc engine versions cause those engines were also in American cars though the 2000cc engine is far more common. I've also found English websites that are easy to order just about any parts you want for a Rasheen including body panels.

Amazon is also nice for finding parts, I was able to find parts for a SR18DE engine on Amazon and that engine was never sold in America. So you can just buy the parts yourself then take the car to a local mechanic for the work.

Once you find something that interests you just Google that car name parts and you can usually find someone talking online about how owning that car has been for them.

The best listings also have video of the car running so you can hear if something is off with it.