Hi
BinaryRS, and welcome to GT Planet!
This sort of question gets asked a lot here, i.e. "Where can I find xxxxxxx car?" or "When does xxxxxxx car appear in the used car lots?"
To this end, two of our extremely helpful & diligent members put together spreadsheets that'll answer all of these questions once and for all!
Since you're in North America, you'll want TornadoAlley's NTSC version which you can download from this post.
It's really nice, fully searchable, will tell you when to find a car, how much you'll pay, what the mileage is, and what colour it'll be! Adjust the cycle number in the top right corner to match the cycle you are on. (Cycle is 700 days.)
The Skyline you're after is pretty rare, appearing only 3 times, for 7 gamedays each time, during the 700 day cycle. In cycle 1, it appears on the following days:-
372 - 378
512 - 518
617 - 623
Hope this helps, and don't forget to leave
TornadoAlley a comment & reputation points (click the red check mark at the top of his post) for his enormous efforts in compiling this info. 👍