I don’t know what happend, but I can’t craft or find the crystalline golem empty arm. I have a few crystalline golems, so I know it can be done. Just, something changed. The crystalline golem empty arm is not in the Golem Workbench selection of items to craft, like it says it must be.
I’ve tried the unnamed city, and I only found other parts.
The Crystalline Empty arm has never been craftable, you have to find it in a drop or purchase it from the vendor. None of the “base” crystalline pieces are craftable but once you have the “base” you can craft upgrades for some of them.
Alright, so I should stay in the UC and consider I just got bad rng returns so far.
Pretty much yes. Crystalline pieces are drops only. You can only craft the tool/weapon part on them.
Crystalline Golem parts can not only drop, but also be found in all kind of loot chests, even the basic wooden ones. No need to stay in UC either, this can literally be in any camp, even lower level Darfari camps and such. Sorcerer NPCs usually have a quite good drop rate of parts (all kinds) as well.
I mainly get stone legs from sorcerers. I think I need to make a bunch on stone golems. Because it takes a lot of time to find the crystalline empty arms. If I had to train them for experience, this would suck. I’m glad golems are a bit like old conan exilies where you can place-and-go. 
I’ve also been looking for one empty crystalline arm for weeks - and have looked in all chests I find and sorcerer drops - nothing. Other crystalline parts no problem, got spares. RNG? Maybe. Coincidence we’re both short of that particular part? Maybe.
My unlucky streak of getting the empty arm drops came to an end. I had about 2k+ on me to buy them. 
I have piles of arms, torsos and heads. What I’m missing are legs of all flavors.