For Azaleas
more mature trees are available from these three sites.
Soh Ju En Satsuki Bonsai - they are in California, and sell azalea in middles stages of bonsai training. They do Mail order, and are competent at shipping.
http://www.sohjuensatsukibonsai.com/
Satsuki Bonsai-en - Satsuki by David Kreutz - David is in the St Louis area and pretty much only raises Satsuki. He imports trees from Japan pretty much every year. Has Satsuki in all price ranges, less than $50, less than $300, Less than $900 and some in the much higher brackets. Email or call David, if he doesn't have what you want he can look for it next time he orders from Japan. Nice guy. Very knowledgeable. Trained under Gondo and others. I have bought many times from him.
http://www.satsukibonsai-en.com/
North American Satsuki Bonsai Center - this is the decedent nursery in the 2nd or 3rd owners since the passing away of the original El Dorado Bonsai Nursery. El Dorado was the nursery that the Japanese trained and certified Satsuki Master Gondo used to teach at. The current owners initially trained with Gondo. They sell azalea at various stages of development, and offer classes too.
https://nasbc.com/
In order
Obtained from David Kreutz, Brussels, David Kreutz