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.
Consider it as just stocking up for a rainy day. Doesn't seem so bad then. In less of course the majority of items are perishables. Then it's preparing for hibernation. Oh, but it's approaching prime speedos season. It seems you may have over done it afterall.
While I'm sorry for you I wish some would fall here. We are very dry. Hasn't rained in weeks. Some localities are placing water restrictions and there have been a few wild fires.