A great place to shop for gluten free groceries on Long Island is The Diet Shop located in Ronkonkoma. They are open 7 days a week and have quite a large selection of gluten free products!
Some of my favorite items that have been featured on this site and are available at their store include products by:
*Bakery on Main
This is only a brief list of the many brands of gluten free products that they carry. If you can't get to their store, then you can also purchase these products on the web at www.thedietshop.biz.