A bunch of them sponsor this site, so you can check ads. There's also the livestock selling/trading forum here and many people ship.
Dying in transit IME is usually caused by a delay (storm, shipping at a busy time of year, or you live out in some forgotten wasteland like Eastern Montana). Extreme heat or cold can be a problem, but if it's well packed, the coral can usually survive overnight. But it can't survive more than 24 hours if there's a delay. My (limited) experience suggests that extreme cold isn't often a problem for 12 hours since they usually come packed with heat packs. But extreme heat can kill corals, whether in a tank or the back of a truck. So winter is a better time to order than summer.