Ok, assume you already know that most are grown from tubers rather than seed, right, and since this is the Saving Seed forum, that you are talking about saving your own seeds rather than buying seed?

Getting your seeds are possible with dwarf varieties especially. They are found in the dried flower heads IF they were pollinated and then allowed to mature on the plant. Dried flower heads can just be placed in a sack or bag, thresh the bag well, and the seeds fall out. Sort them from the chaff and store in a paper envelope till spring. Start them indoors 6-8 weeks before transplanting time and treat them as annuals. Be aware that many hybrid varieties will not breed true.

Hope this answers your question.

Dave