Coolant as in ethylene glycol or propylene glycol is a less effective cooling medium than is water. Water is not a very good water pump lubricant, and tends to freeze which can cause the engine to crack. The coolant is added to lube the water pump and lower the freeze point.
As mentioned, premixed coolant is readily available in most places, so I'm not sure how you missed it. You should be able to find it at any automotive or powersports store. Make sure it is safe for aluminum engines.