Most tackle shops have them, however there has been a phenomenal run on them lately ;-)
Keep them in a five gallon bucket of sea water that you continually recycle. Yes, some die, but once on the hook in the water column, they give a live presentation; we've caught on "dead" ones before, more than one Striper too.
We normally get ours at Oceans East 2 on North Hampton. Don't understand the run on eels lately. I normally won't fish with a dead eel but have used the same eel on three different fish before he finally died. Here is our eel fishing results from last weekend.
The best thing I have found for handling eels are those 99 cent brown garden gloves. They are cloth. Just slip one on grab the eel and hook him up. Take glove off when done. The only problem is after about 15 baits the glove gets slimed up and you need to change it. Of course a trip to the washing machine well get them back in shape and ready for duty for the next trip. I buy two pair a season.
Danny's right, never put them in water. Go to the dollar general and get a six inch deep tuppaware or similar enclosure, put a wet hand towel in it, along with your eels.NO WATER!!! Set this down into your big cooler.Take along a couple of green scotch brite pads. Have your hook in your hand ready to go and grab him hard with the pad behind the head and run the hook through his bottom jaw and out one of his eyes. After he takes a tick or two you can reehook him through the other eye. Take six 2 litre bottles and rinse them out. Cut off the tops where the sides meet then duct tape them together. Ease your eels down into the bottles while under power of the big motor to prevent them from tangling.
I kept 14 alive in a 5 gallon bucket for 2 days with about 3" of water in the bottom. All were lively for the second day of use. I did not ice them they stayed in the garage at night to keep them from freezing.
Haven't stored any for more than 2 days this way though.