When you DO buy replacement or spare plugs, get the good plugs, not the cheep plugs.
Just like props, you will get a noticeable difference in performance between manufactures but with glow plugs that also means longevity.

I used to buy a half dozen $2.95 plugs at my LHS and would go through them in six months, and now a $6.95 Enya #3 or a OS F Type plug will last me that long in 2 or 4 stroke engines burning Powermaster YS-20/20.
Remember, you can use 4 stroke plugs in 2 stroke engines but not the opposite.

Experience is a wonderful thing.

P.S. A good friend will keep you from doing stupid things... alone.