There would likely be tons of sites with temporary storage wings, since there's no reason to have only one temporary-holding facility for a worldwide shadow organization.
That, or:
- They have dozens or hundreds, each serving specific regions or resources.
- They outsource all of that crap, and buy and deliver materials though middlemen or front companies.
- All of their resources are mined from asteroids/traded from alternate universes/summoned by blood sacrifice, and are transported directly to major sites through SCP-120/Multi-Universal Transit Arrays/bloodstone circles.
- There's one, and it's under Montana. All of Montana.
I'm not trying to be dismissive, I legitimately canon the multiverse thing. I'm just saying, things are exceptionally flexible, especially in the mundane bits people don't tend to scrutinize. If you're writing it, it's pretty much up to you.