I don't know about petunias specifically, but on some double-flowered plants, the reproductive organs are sacrificed, so to speak, by the creation of additional petals. So, on some double-flowered plants, if you can't find seedpods, it's can be because the plant isn't capable of producing them.
Thanks so much for the link it really helped . I cant find a seed pod on my doubles so maybe they cant set seed , I wonder if I can get like a cutting off of them and put it in root hormone to save them for next year? any one tried this?