There's certainly no shortage of pollen being shipped around the world between growers, but I haven't ever seen anyone selling it openly. Much, much more volatile product than seeds. I've heard tales of people successfully using frozen pollen that was many years old, but it's definitely best when it's fresh. It can become non-viable within weeks/months of harvesting it, so it's not the kind of thing that most retailers are going to be eager to stock in inventory. Then you have the issue of shipping and maintaining optimal temps until it reaches the buyer. Not an impossible task, but seeds are just a hell of a lot easier to deal with.