I've ever only seen them at the seafood monger I use when we're in SC. Then they are only seasonal because they're live. You might be able to mail order frozen ones but that would likely be prohibitively expensive.
They are but there's some sound advice with the nails. Each provides a neccessary element often missing in soil. The sap slowly leaches the material from the nails and nourishes the tree. Of course if the tree is small, ie shrub sized, I would insert the nails into the soil at the base of the tree not into the tree.