One of the best places to start is partzilla.com. Partzilla makes it easy to find part numbers for your model and cross reference other models that part fits. You can also purchase new OEM parts from them, when available.
Once you have a part number, you can enter them into a number of sites to find the best part for you at the price point that works. These are some of the most common sites.
Z1Enterprises.com is a proud supporter of this forum and has many parts for your bike. They also have excellent customer service.
Z1parts.net is able to fill in a lot of gaps, and sometimes get you parts cheaper than you'll find elsewhere. They have very quick shipping.
DiabloCycle.com has a few bits that I haven't been able to source anywhere else, like grips and decals.
redlinecycle.com/ Jim's website doesn't have your typical shopping cart, but he does have lots of parts.
www.johnnysvintagemotorcycle.com/ Johnny's is often more expensive than most, but he specializes in '70's Kawasakis and has a vast collection of parts for sale. He's also worth looking up on YouTube. You might learn something.
Ebay.com, ebay.ca, ebay.co.uk, etc. This one is pretty obvious, but especially if you know your part number, eBay is a most excellent resource for the hard to find parts.
Parts n more www.partsnmore.com/ is also a great source for hose clamps. They have more different sizes and finishes then I have found anywhere. They list the clamps in ID size which makes it idiot proof to match to hose size.
Dennis Kirk has read deal Mikuni jets and float valves for good prices. Regarding float valves, Kawasaki sometimes speced sized in between the common Mikuni sizes. For example, they spec 2.3 when Mikuni offers either 2.0 or 2.5. When in doubt go down in size. Suzuki GS bikes use the standard Mikuni sizes and those bikes typically use the smaller size float valve (2.0) yet they make just as much power as a similar KZ so that should tell you something.
82' KZ750-R1 cafe racer style. Clip-on, Rear set, Fork-Brace, Mikuni Flat-slide (forgot the size), Kerker.
84' GPz750 (basket case). everything are in pieces.
89' ZX750-H1 (ZX-7 Ninja). Resurrection project are on going with my KZ750. Everything is stock.
Addition: I shopped around a little and was happy with Orient Express for Mikuni RS34 carbs. Their online catalog seemed to have a lot of Kawasaki high performance parts.
Former M.E. at Kawasaki Motors Manufacturing, Lincoln, NE
1966 W1 (the Z1 of 1966-50H.P. and 100mph!)
1974 Z1A (disassembled)
1976 KZ900B LTD (SOLD!) it's in GOOD Hands!
1978 KZ1000 LTD
1976 KZ900B LTD pile O parts!