I try to keep a complete set of spare parts for my residential oil fired furnace.
Recently, though I needed a new ignition coil for my furnace. Then I remembered that I helped the ex-inlaws, about 10 yrs. ago when they needed a new ignition coil for their furnace.
Fortunately, I was able to locate a new ignition transformer for my furnace within a 25 mile radius before 5:00 PM on a Friday.
Moral to the story: keep your inventory up to date. Eventually old furnace parts will be hard to find.