it depends on your driving, if you constantly have flats, blowouts, etc you will be better off with a local dealer like discount tire or other large chain for service.
if you happen to have better luck or experience few tire problems tirerack.com and your favorite local tire mounter would be the most cost effective.
tirerack carries the best selection for the price. most of the time, it will take just as long or even quicker then your local store ordering specialty tires. you might get away with ordering from a smaller online store and wait an extra 2-3 weeks for less money.
i have ordered a set of 4 bridgestone potenza so-3s and a set of 4 yokohoma avid sts, as well as single tires from tirerack. delivered always as promised and saved enough money to get some decent tires, thats counting all taxes, mounting, shipping, misc shop fees...