Hi mate,
I've not bought a kit from the US myself, you could go to a local person to have them done. I think Sulio (Toecutter) did this and got great results for his 260z 2 seater, not sure how close the pattern matched to original though.
Datsun changed the seats, material and stiching used over the years, early seats were criticized due to wear and tear issues and Nissan changed the pattern and material used as the years went on to higher quality material and stitching (although styling didn't improve in my opinion).
For that reason I'd say having OEM material might be nice, but I myself would be happy with a quality aftermarket kit provided it was high quality and close-to-original. The seat cushioning will more than likely have sagged over the years so seat skins will probably not be enough to properly restore them (actually the expense was the reason I went with aftermarket Recaro's in the end, but I still have the seats if I want to recover originals later).
Then again if your building your car for concourse you may have to go the extra step and go genuine Nissan.