Great Thread Topic!
Here are a few companies I have purchased from in the past.
DrMacCorals.com
They usually has a wide variety of acropora in stock. Frags are usually a very decent size. Shipping is expensive from Dr. Mac. My shipping costs ran $65 + $10 box charge. The package should arrive before 10:30 AM with FedEx Overnight Priority service and pickup at a local FedEx office is recommended by Dr. Mac. My corals arrived packaged very well with each coral in numerous bags with a fair amount of water. Good customer service as well. I would recommend them to others.
http://www.drmaccorals.com/
eTropicals.com
Good service. Decent size frags. I would recommend this site as well. Low shipping rates.
http://www.etropicals.com/
FloridaPets.com
Great company to deal with. Excellent customer service. Website has a good selection of macro algae, stars, snail, crabs, and various other critters. Highly recommended. Sometimes carries hard to find items.
http://www.floridapets.com/