Well I bought it from a friend. I assume I need to get a water testing kit and other misc. items. I just don't want to miss out on the important things.
A good one will have them. One that specializes in marine aquatics would be most likely to carry them. It is an accurate way to measure the specific gravity of you saltwater(salinity)
If you can't find one at your fish store, you can find them online for $40 to $50. You use it instead of a hydrometer to measure salinity. Hydrometers are inaccurate and have to be replaced every 6 months or so. So you make up the cost of the refractometer in a year anyways.