Lots of choices shopping for night vision. New, used, fake, or worse - stolen military gear. There's legit used gear out there but don't buy questionable gear. Not only are you screwing over America but it could cost you a visit, confiscation, and/or worse.
Any of the good stuff is expensive. Get over it. Don't waste your money or time with anything less than Gen 3. People on web forums aren't always looking out for you (duh, right?).
BUYING NEW YOU KNOW WHAT YOU'RE GETTING
There are a lot of night vision devices online. Generally, the easiest way to gauge the performance and quality level is pretty simple. Cheap = low performance and Expensive = high performance.
There are only two manufacturers of US Generation 3 Night Vision. Harris/Exelis (formerly ITT) and L3. They supply the US Military, our Allies, Federal, and local Law Enforcement with night vision devices.
There are many dealers and sellers who offer both. For Harris tubed devices a dealer we recommend is JRH Enterprises. For L3 the biggest name is TNVC (Tactical Night Vision Company). TNVC also sells devices with Harris tubes and NVD devices.
Buying a new device from a good and reputable dealer guarantees you'll get a new tube with good specifications and warranty. New devices should always come with a spec sheet from the tube manufacturer. This sheet lists the actual performance of the tube as measured using test equipment.
Buying used you may or may not get a spec sheet and even if you do, you'd need to verify the tube serial number to make sure it matches. Military gear don't have spec sheets and this is the reason why you'll see Omni VII, Omni VIII, and such used to describe them. Omni refers to Omnibus and the Roman numbers to the Omnibus number. These are military spending contracts which include specification requirements for generation 3 night vision. Over time these specification numbers have changed but every device & tube has to meet the spec, so this is why it's used to describe a tubes performance. A good source to explain the contracts and the specs is on the ar15.com night vision forum. Members there have spent a lot of time compiling this info.
NVD used to be the only company that offered a 10 Year Warranty on Harris tubed devices. Other companies such as TNVC have followed suit. This a win-win for civilian night vision users/hobbyists.