The rice paddy herb is used in Vietnamese cooking that imparts a sharp citrusy flavor. Look for the herb in Asian markets. When we tasted the fresh herb it was like tasting fresh lemon zest. The herb is usually chopped and added fresh to soups and Thai curries before serving.
Andrea Nguyen has a very informative page about growing your own rice paddy herb in a mini-greenhouse that just uses a plastic pot and a large plastic bag. We found several stems with roots that would probably grow if you placed them in water. That would give you a plant start. Otherwise look for small six packs at local Asian markets.
This herb is hard to find but look for it in Vietnamese grocery stores. Lion Markets carry a nice selection of Vietnamese herbs.