Whatever is available in your area and is intact enough to sprout. Some people get whole grains from the bulk bins at the grocery store, others buy bags of seed from the feed/farm supply store (can be feed grains or planting grains). Others order from online sources, though shipping can be a killer.
If you can speak to someone who knows something about seed and can tell you which seeds will germinate (sprout) but have NOT been treated with anything, that would be a big help. Lots of seeds are treated for one reason or another (feed and planting seeds both).