There are 376740 such sets and you must think me crazy if you think I will list them all!
There are 6919 sets
It is approximately 1:750000 so approximately 177 sets of quads are born each year however many of these are reduced to triplets or twins due to complications.
Quads can be found for sale in a great many places. For used quads, specific automotive magazines or websites carry a large variety of ads for these vehicles. New quads are found at the manufacturer's dealerships.
around 50,000 quads are made in the US every year
54 quads is 163.99.
There are many different types of quads out there. If a person is looking for a used quad Craigslist has many to sell options, or the classifieds in the local newspaper.