Given the fact it is a 10+ year old car, some are readily available and some arent. You may find some people who stocks bits and peices like radiators, fuel caps, oil caps, rad caps and other little things. Performance parts are generally OK in terms of supply.
eBay has alot of non-performance parts for skylines, performance shops like www.justjap.com, JDM Yard and other Japanese Performance Parts (you can google that) should have stock since people track their skylines and will be in need of replacing certain parts, which creates a market for it.
For things like Genuine Nismo body kits like my one in my sig, may be harder to source, new anyway. Interior trims and panels, electricals and body parts are available every now and then from here on the for sale section of the forum or you can pick something up at your local Jap car wreckers.
I've replaced a few bits and pieces where no one would have, hop over to a wrecker that specializes in Skylines around my area, dudes got everything. I haven't had any issues finding anything so far. Just got to know where to find it from.
Justjap and JDM Yard are Syd stores so it may also depend on whats available locally for you.
Wheels, don't think I need to cover this.