WholeGrainsCouncil.org offers a list of gluten-free grains. The site also provides information on celiac disease, gluten intolerance and where to buy gluten-free foods, as well as resources for gluten-free cooking.
The list of gluten-free grains includes corn, rice, millet and quinoa. The site notes that oats are naturally gluten-free, but contamination with wheat frequently occurs during processing or growing.
There are many companies that offer gluten-free foods by mail, such as Bob's Red Mill, Lundberg Family Farms, Twin Valley Mills and Gluten Solutions.
Federal guidelines require manufacturers to list wheat as an allergen, but do not require listing of other glutinous grains. In 2013, the U.S. Food & Drug Administration released guidelines for labeling foods as gluten-free.