I buy a year's supply of steel and use it until it is just about gone.  6-10 pieces at 6 foot lengths. Then I re-order.
With handle material, I buy 8-10 pieces at a time.  Or I trade, barter, accumulate, just about all the time.
With leather, I buy a couple of sides every year.  Usually at one time to save on shipping.
Essentials like belts, sand paper, epoxy, etc, - I accumulate a list and buy it when needed.  I try to save on shipping costs and order all at once.  Does not always work out, unfortunately.