How about having "wild card" materials? Like if you need a certain material for a piece of equipment you could put it there instead. Put a limit on it for sure..... one per piece maybe. Example : I've needed a legendary Fur Tuff for ages , literally months, but do you think I can get any? lol a very very few of the simples.  With a wild card I could use that instead. You don't have to make them easy to get or even super cheap to buy but..... I'd buy a few for 5 or even 10 dollars apiece.