I purchased my 5 gallon Carmen (on Dusa rootstock) from Persea in Fallbrook CA. They sell to the public but very informally, it was basically call and tell them what you want and meet at the front gate with cash. I think I paid somewhere around $35, also snagged a GEM. I doubt they ship or anything like that, they seem like a supplier for large groves and just sell to the public as a courtesy. You can try contacting them though, their website is www.avocadonursery.com

Also I think Brokaw has the US patent on this variety but unsure if they sell to the public.

Regarding Carmen, very interesting tree, mine has only been in the ground for a couple months but it has flowered wildly and even set several fruit, all on a scrawny 4 ft tree. Looks like it will have multiple fruiting seasons. Seems like a Nov-Dec offseason crop in addition to a normal season Hass crop. Hard to tell what the growth habit will be for the tree but I have read it is suppose to be compact which is how I spaced it.

Good luck!

Yeah I would need mail order