So first, I have to thank you for all the effort put into this thread, even inside a country, there's usually enough disparity to cause a little chaos in our minds at first glance. I will add inn my experience from Europe based on Iberia...
My base mix uses store bought soil and compost/ewc. I do think they're only making biz in POR/SPA, so not much of a help for here. If you're in Iberia do a search for SIRO, the "aromatics" mix and try a bit, lots of ewc, no sterile aeration, etc.
I try to source my things around either bought or collected. Having said that when I really need I have a nice hydro-store kind thingy in the country (I think they send abroad too) that has more than one kind of guano, lots of other amendments (neem meal, kelp, fish,guanos etc) that is urbicult-pt.
Many other stuff that I buy comes from agriculture shops, animal feed stores, etc. It took me a year to finally give in and do like others, search in the real world for alternatives.
- Oyster Shell? Try it in the pet store or animal feed, it's used for grit in birds remember?
- If you want mulch and don't want to colect/are afraid of pests, try again the animal store for bunny/rodents litter, it's usually made from wood shavings and comes compressed as coco-bricks (dirt-cheap too cause it's a by-product).
-Or alfalfa for example. The country where I live doesn't have so much alfalfa/lucerne) produce per year. Reasons apart, you may find it in straw for animal feed, or seed for cover crop.
- If you want rock dusts go to/contact places that cut, separate stone, etc.
And that's about what I remember for now. Cheers