Re: Home Depot/Lowes mangoes: size depends on when you buy them and when they restock them. I've seen some relatively large trees.
Laguna Hills Nursery has some grafted mangoes -- I saw an Edwards there a few weeks ago.
Champa nursery in El Monte has lots of Florida mangoes mainly from Pine Island Nursery.
H&H nursery in Lakewood has lots of LaVerne nursery mangoes
HD and Lowe's have a much more limited selection, but you can order various LaVerne mangoes from them (I'm pretty sure).
Good luck. HD/Lowe's prices are 25-40$, 70$ for Keitt. Same with H&H. Champa and Laguna Hills will charge more but you can get other varieties from them.