Let's say there's a school club. There's a person in charge of ordering/buying things for the club. What would you call this person's role?
Most clubs that I've known assigned such a role to the person responsible for the funds, so "treasurer" is quite common.
"Purchaser" and "buyer" are another clear approach. I've only seen these as roles in a business context, but a club certainly could use the terms and be clearly understood.
For clubs or societies, you should research the role of the quartermaster. This might come closest to the duties and responsibilities you describe. Also used widely in organisations like scouts and guides.
A person that helps to buy things could be "an advisor".