most garden centers will have a good selection, and they will know what will grow well in your yard. some farmers markets will sell them as plants too if you want to buy local
other herbs will sprout on there own in water (like ginger) and you can buy them at grocery stores.
if your a beginner i wouldn't start with seeds they can be fickle depending on the plant. i would get a bit of mint and find a nice little plot (sectioned off from other parts of your garden or in a pot because it likes to spread) because its a great beginner plant with tones of uses. work your way up from that with thyme and basle and the like.
some herbs (like ginseng) will be harder to find to grow, but you can buy them at most places that sell bulk herbs