Don't risk it trying to use some random other type of coolant. We've always found the best to be the original Toyota factory fill stuff! This is not diluted with water, so it is excellent for topping up your cooling system a bit, or easily mixed to the ideal mixture for your region.
If you live up north or anywhere that it snows, we recommend keeping a 50/50 mixture or so in your Supra. In southern states that don't see freezing weather, you'll often see people running only 35% coolant in their system and the rest distilled water, for the best performance.