Curt, the best distributor I have found so far is Motorsport Auto. Even though it doesn't say Z in the name, that is all they sell. They have a very good selection of carpet kits, they do have leather seats as well. I bought a seat upholstry kit from them last year for about $200 for both seats. The kit was exactly like the original vinyl upholstry pattern, except the kit is made of superior than original materials. You could go leather, but I'm not sure they sell original replica patterns in leather. THis place has everything from engines, dashboards, even heater knobs and consoles. Check them out, they're great. www.zcarparts.com/
Liam is right, Motorsports Auto is a great place. I like them so much, I wish I could buy their stock. Recently, I was looking for a carpet kit too...(73 240Z) and found out that you can shop around... but I couldn't beat their price unless I went used.
Ask them to send ya a catalog and price book, I think its better then their webpage.
i agree that motorsport is a good source, however for what you want, i would recommend bonzai (spelling?) motorworks out of Maryland, they have carpet kits with the original stitch pattern and also offer leather seat kits which i believe are in the original seat pattern. i think their site is www.zzxdatsuns.com.