A lot harder than just buying one. Manufacturers have it down to a science, and they can make springs with any level of resistance they need. It's a lot harder than just turning piano wire on a jig and heat treating it after though this may be possible for things like magazines.

Glock doesn't make their own springs, they get them from a vendor that specializes in manufacturing large volumes of springs. It makes no sense to tool up for something outside of your core competency when specialized firms exist that can sell them springs for 25 cents apiece by the tens of thousands that work because they have QC and material science from decades of experience.