I ask myself those same questions Tiffany. I saw a wonderful double color (it's posted here in plant files) one year and was so excited you'd have thought it was Christmas!
I truly don't have a clue where the rare ones are. Grandiflora, umbraticola, oleracea may be the big boys but there are others. Maybe a google search?
Check this one out!
Double Portulaca (Portulaca Fairytale Cinderella)
