First, I have spent many many many hours looking over parts diagrams and tracking down oddball Z1R parts. If you have a list of things that you need to find, I can tell you where to look and how hard it might be to find.
Another note. The front calipers on the Z1R are actually different than most others from that era, but they look identical. They use a smaller piston, so the normal kZ1000 rebuild kit might not have the right seal. And if you need pistons, make sure to get the right ones.
Also, this thread might be useful:
kzrider.com/forum/5-chassis/569907-z1r-f...build-kit-substitute
For OEM parts that are still available from Kawasaki, I use
Partzilla.com
One note of caution. For fasteners, many of them are still available from Kawasaki, but they often don't look like the original part. It's equivelent, and will work fine, but just doesn't look the same.
For used original parts I use ebay. I haven't found anything better. There are guys parting out whole Z1Rs, so when those come up you can often find almost any part.
If you're looking for a part on Partzilla and it's no longer available from Kawasaki you can often find it new on Ebay by searching for the part number. There are folks who drive around the country and buy out old stock sitting on dealer shelves, then list it on ebay. They apparently have stock that's 40 years old on the shelf. I met a guy in Austin who did that for a living. He would cold call dealers all over the country and offer to buy the entire inventory, and then drive out to wherever and pick it up.
But a note of caution. Again, mostly with fasteners, Ebay sellers know that the parts don't look like originals. So they will put up a picture of the original, and send you the newer, different, part. But the part number matches, and they are selling the genuine part, so they aren't exactly lying.
For maintenance parts and some reproductions, I use
www.z1enterprises.com
.
z1parts.net
is another. There are others, but Z1 Enterprises has been around for a long time.
For really generic stuff, I still use DennisKirk.com. Mostly for tires.
For some fancier reproductions, I use japan.webike.net. There are Japanese companies that make nice reproductions of some parts as well as very expensive upgrades. Doremi and PMC are the ones I know best. Some of these Japanese reproductions are sold domestically by folks like Z1parts.net, but they often don't tell you where they are sourced from.
There are some reproductions sold by US companies. Sudco makes some Z1R parts.
Finally, as Nessim mentioned, it's important to know which parts are common to all KZ1000 models, and which are Z1R specific. A lot of parts are shared. It used to be much easier to figure this out, but Kawasaki changed their on-line parts database. If you look at part on Partzilla, it will show you the other models that part is used on. But the data is lumped together for similar models and years, and it often lists models that do not actually share the same part.
For example, the 18" front wheel is only used on the 1978 Z1R. The 1980 Z1R went back to 19". But look at this page:
www.partzilla.com/product/kawasaki/41073-1037
In the "Related Fitment" box, it shows
Kawasaki Motorcycle 1980 Z1R - KZ1000-D3 FRONT WHEEL/TIRE/HUB ('78 D1)
Kawasaki Motorcycle 1978 Z1R - KZ1000-D1 FRONT WHEEL/TIRE/HUB ('78 D1)
It seems to list both the 1978 and the 1980. But if you notice, both of those lines say ('78 D1) at the end. Confusing.
