IT Purchasing

I'm working for a large company that doesn't have any real purchasing processes. It means that IT get asked to buy everything and everything, from shredders, printers to use at home, projectors, foot rests, earplugs (yes really).

I'm trying to define what IT should buy, and what we can allow individual departments to order themselves. We desperately need to get some good processes set up. How does it work in other large companies?

I think we should buy laptops, mobile devices, external drives, and software. Should we be too worried about anything else? Keyboards, mice and monitors are already handled by another team.

We also get a lot of requests for accessibility hardware, such as streaming devices for hearing aids and magnifying devices. I don't think IT should be purchasing these items because they're quite specialist and we wouldn't be able to support/maintain them either. Plus it means knowing all of the employees health details which isn't for IT to be involved in. Who purchases this type of equipment tech where you work?