I'm currently using "Paws Gourmet Soft Training Treats", Kickin' Chicken flavor.
I just checked the ingredients and they include Oat flour, so they may not be for everyone.
I think you can pretty much use anything for a training treat, as long as it tastes as good or better than the dog's regular food. It should be small enough that the dog can quickly eat it and won't fill up before the training session is over. You should also keep in mind how much of the treat your dog is likely to consume during the training session: too much fiber or fat could upset the dog's stomach, too much in general may make your dog fat or make him pickier at meal times.
I used to take a slice of meat from my lunch sandwich and cut it into little bits. I've heard of people using apples, carrots, green beans, cheerios, turkey, chicken, roast beef, hot dog bits, cheese, and even the dog's regular meal. Some people feed part or all of their dog's daily kibble during a training session.