Hello and welcome aboard!
Unfortunately, to my knowledge the G25 models are few and far in between in the States. And I would assume the people that bought them these sides were interested in the G body, RWD and a modest fuel economy, rather than crazy performance.
If you still have the option and the means, I would do my best to get the VQ35 or 37 engine. As you are finding out, those have vastly more modding support. Otherwise, you'd be pioneering... or shoot Stillen an email to see what they think.
Then I am also aware of other options like HKS superchargers or whatever the Japanese brethren have been doing, so you will not have to be limited to shipping from the States.
Unfortunately, that is the extent of my knowledge on the platform, so I'll move your topic to the G subforum for more knowledgeable responses: