There are plenty of threads in the past here regarding stripping down these models - the Zero 100 was a refined version of the SL75B and 95B and had lighter and more delicate trip parts to start with, to go with the posher looking tonearm (I think the 75B and 95B were refined later along similar lines, but my 95B decades ago was a very late one and I've not seen an early 'B' type underneath). Where with Dual, it's recommended to strip out fully, I'd say with these Garrards it's all but essential, as the greases harden badly, especially around the speed change mechanism and start-stop lever under the platter and riveted spindles can rotate and fine parts break if they're forced. My Zero was very badly gummed up but came back together beautifully, but you'll need patience, also in getting hold of the replacement part you need.
Regarding tonearm parts, try South Street services - you never know... Otherwise eBay sometimes comes up trumps.
First though, you need to look at the service and owner manuals here, carefully remove the platter (the mat should come unstuck easily - make a note which way it comes off - I can explain later
- and the fixing clip can then be got at to check the idler, motor spindle and main bearing as well as the often stuck start-stop lever. I have some pics in my gallery showing my partly dismantled AP76 and in many important respects it's the same as the Zero.