When I lived in California and had to move East, I wanted to cultivate some of these after the move. I couldn't find seeds in them either so I dug up a small plant and discovered they grow from rhizomes - like Irises - similar to a bulb but more like very fat roots. That is why you see them growing in "clumps", the rhizomes spread out from a central spot. They are beautiful and interesting plants.

If you search on Google using the string "bird of paradise seeds" you'll get quite a few links selling seeds. Here's one for example - http://www.hawaiian-tropical-flowers.com/birseed.html

The seed will come from the flower if it is pollinated (sunbirds do it in their native habitat) otherwise you have to hand polinate them to obtain seed. http://www.clemson.edu/charleston/local/bird/strelitz.htm

If you're waiting for seeds you're on a losing battle. Just make divisions of the growth rhizomes from the backbulbs. Then using rooting hormone, and you've got new strelitzia