I've been using Z1 Enterprises, Z1 parts.net, Johnny's Vintage, Partzilla, and several UK and German and EBAy outlets for my Z1 project. I got my carb holders (carb to head) from Z1 Enterprises, they seem pretty nice but have not been installed yet, I'm not that far along. Parts for early Z1's are a little harder to find in some cases. There are a few carb parts that are REALLY hard to find for the early ones. Don't ask about cost.
My Z1 came with a real mixed bag of fasteners, almost none of them original so that's been a real challenge.