I know you said you wanted large specimens, but I would
suggest Worldplants (www.worldplants.com/mapleintro.htm) and also try a few one-year or two-year-old japanese maples. I purchased 6 from them this spring (Bene Maiko, Shin Deshojo, Ichigyoji, Orido Nishiki, Osakazuki, Hogyoku)because the site had gotten rave reviews from so many people. I wasn't expecting much in a 4-inch pot - maybe a crummy-looking twig - but I was so pleasantly surprised when I received in my 4" pots VERY healthy-looking young maples about a foot high and nicely filled out. I'm growing them in 8" pots until fall (when I plan to plant them in the ground after their leaves drop) and they are doing great. Worldplants has a lot of new hard-to-find varieties, too. Large specimens are nice if you can afford them - but I'm having fun with all my cute "babies".