There are two main choices, a hydrometer, and a refractometer.
Hydrometers range in price, from really cheap plastic ones(5 bucks or so), to more expensive glass ones - as with any type of precise measuring tool you get what you pay for.
Refractometers are more accurate then hydrometers, but more expensive, I have seen inexpensive ones for 65-70 dollars, on up to several hundred.