Neither WalMart nor Ace Hardware. If you are firmly convinced that you need to add it, because you are getting summer #2 diesel, then you are going to have to find a service station that pumps kerosene. You will need to add a decent amount (20% minimum) to have the desired effect. So, if you have a 20 gallon tank, you will need at least 4 gallons of kero. This is not something that you want to buy at a hardware store.

I do believe that you are getting ahead of yourself. If the glow plugs are not working properly, the engine won't start on diesel fuel and it won't start on the diesel/kero blend.