Actually, I'm not really sure where the original herb seeds sprung up. You can't buy them from vendors. Community herb gardens would be a good idea, but you should ask first. And there are a few on the auctions.
I'm actually interested to know as well... where did the first herbs/herb seeds come from? Besides the quest, which was only added recently, the community herb gardens, and auctions/trades/shucking, could you only get them from dust traps?