SA Gas-blocks are about as easy to acquire as complete R4 assault rifles.
I've joked with a few people that asked for them -- the only way you will get an SA gas block is to take your closest Johannesburg police officer's rifle.
The problem is not that they are serialized components (you wouldn't believe how many guns 'on the books' go missing and are never found), but the fact that they do not survive the de-mil process and are not ordinary spare-components. For example, there are many handguards and pins because these will either become damaged or worn over time and need replacement. If you have a worn-out gas block, cracked receiver or shot-out barrel then you just ditch the whole rifle, hence why these parts are unobtainable.
My understanding is that the only parts that survive de-mil and become surplus are the ones that can be removed during a quick field-strip or with a hammer. Again, this is why you never see front-handguard retainers or gasblocks or magazine-releases since they are permanently affixed to the weapon. The 4 SA blocks I acquired were all specifically saved during de-mil (that's why they were on barrel sections), something that doesn't happen very often.
If you can find an SA block, you will likely pay a fortune. I'm aware of SA civilians/police with LM4 rifles that cannot get the blocks regardless of the price.
Your best bet is to just wait for Jeff to make his reproductions of the type-1 ARM block. It could take years before he gets around to them, and hassling him won't make it go faster, but I trust he will get them done and done right at some point.