The small "silversides" are sold at any Asian market, both frozen and defrosted. Same stuff as at an LFS, but much cheaper. They frequently sell the real silversides, Menidia menidia, from local waters. Those are slightly larger, 2-5"...but they carry them friesh.
Spearing are the same fish as silversides if i'm not mistaken.
Go to a bait store and get a pack, they're probably much cheaper. Or get a seine net or any net and go down to the beach and catch some yourself, theres plenty for everyone.
You can buy them at Canal St. Fish Market in Manhattan for about $2 per lb.
There are two kind though but they all are about 3'' and have a distinctive silver line on their sides.