Most anti freeze works as a coolant as well. You can get it from Car Spares places. There is a correct mix, coolant and water. Read the back of the bottle, it will tell you. If you are just topping up, water would suffice. Probably best doing a full coolant change before winter sets in, that way you know that you have the correct amount of anti freeze in the bike for the nasty cold weather.
for winter you normaly want a 50/50 mix of antifreeze and water - that should be strong enough unless we start seeing below zero teperatures over winter!!! - lets hope not eh