The metal-rimmed headlight covers were a Japanese market option in period; that's where they can mostly be found, but as you note, they aren't cheap! Used originals or repros, in both cases are multiple hundreds of pounds.
As Geoff notes, Rock Auto in the US is very useful for standard mechanical parts - swift, and relatively cheap. Other US sources - Z Car Depot, JDM Car Parts in CA for more special and JDM-specific items.
RHD Japan - parts ordering from Japan. A strong selection of parts from manufacturers like Kameari (who you can also go to direct), Fujitsubo, Marugen Shoukai, Cusco, etc. Useful as a one-stop shop for Japanese market performance parts.
Body repairs - you need to check out KFVintageJDM - incredible and growing range of high quality, reverse-engineered, repro body panels. US-based with production in Colombia I think, but has a European warehouse in Spain, which is a bonus. Gerson, the owner, is a good guy to deal with too and a Z enthusiast.
Also body panels - APS, Auto Panel Solutions, here in the UK. Andy Plant, the proprietor, works with Duncan Pearcey of the Z Farm in Thirsk. You can find APS on Facebook. I've used an assortment of his panels in my project build and plenty of others here can vouch for his output. Not the scale or industrial capability that KFVintage has (the latter having full-scale stamping tooling, now producing parts like full rear quarter panels, etc.), but makes key panels for repair areas as well as all-important RHD conversion panels.