I have gotten nice ones from AquaSD. Green/common colors are very affordable (especially during a sale) but if you are looking for fancy colors, their stock seems to be limited and pricey (though I have never really compared them to other retailers in this regard)! That said the haddoni’s, and other nems, I have received from AquaSD were nice and healthy!
It really depends on the color. Petco seems to get the more common colors all the time. If you have a local Petco, get to know manager in the Aquatics section. They can usually order a haddoni for you.
Corals Anonymous usually has them at decent prices but the reviews are mixed.