Ageratum seeds form where the flowers are. They are ready when the flowers look like they've turned brown. Just shake them. They will self seed all over the garden.

I don't grow canna, so I can't help you with that.

Cannas will produce a large bumpy pod. When the pod turns brown and starts to crack open, the seeds will be ripe. Many hybrids do not produce seeds at all although some will produce a pod.