Living in CA, I'm not sure why you would want to bother with any mail order sources - retail nurseries and garden centers abound and they should carry a full line of suitable hydrangeas. And most mail order sources will not ship into CA due to agricultural restrictions.

As to the type, I'd pick any hydrangea variety I liked that would not rapidly outgrow the container. I grow many hydrangeas in containers but most lean towards the dwarf or more compact forms. My favorite at the moment is H. macrophylla 'Horwack' - aka Pistachio - for its compact size and plethora of multicolored flowers. Also moderately sun tolerant.

You could also use any serrata, they tend to be smaller with small and quite delicate looking blossoms. If you want some names, check the Joy Creek on line catalog and then get your plant locally. One lovely hydrangea is Pia, another is Blue Deckle and there are many others. Miss Saori is pretty new and a knockout! Do some homework on line so you know what to look for but at this time of the year I would not subject a new plant to the possibility of sun stroke in the mail. Consider blossoms that are in scale with the smaller plants.

Here is a photo of my Miss Saori. She was the winner at Chelsea Garden Show

two years ago.