The best answer is choose a seed supply house with similar or colder climate than you have. As a resident of the Northeast, I have to say I love Johnny's Selected Seeds. They are located in Maine. They have a wide selection, develop new varieties all the time, have many organic seeds available, and you can use their catalog as a reference for planting and harvesting details. Most important, their seed sprouts every time. They cater to the small commercial farmer, think farmers market, so you can also get seeds in bulk.