Hopefully you have a good local source as they seem to be the best source for larger colonies. I just picked up a 100+ head dendro and have 3 large sun coral colonies, one with over 100 heads and got them all from local reefers. You're correct; most retailers cut them down and sell them in one or two head frags. I could make a killing if I did that with the dendro but couldn't bear the thought of it! There aren't too many corals that can compare to a large sun coral colony, fully opened, as far as looks go
If you don't have a large, local community; I would search your closest, major, city on Craigslist. A pretty good source for them, imo