Most herb festivals around here are in the fall. I don't know of anything in the spring. You might check with Mountain Home, Ark Chamber of Commerce, or Ozark Folk Center in Mt. Home. There might be an herb club in Springfield, but none other in Missouri that I know of, our local herb club dissolved many years ago. There are some garden tours locally, but not usually until early summer. Maybe someone else has heard of something.

Baker Creek (rareseeds.com) will have their Spring Planting Festival the first Sunday and Monday in May. It's not an herb festival but there will be vendors there selling herbs and there may be speakers on the subject. I didn't see any kind of schedule of events on their website. I guess it's too early. I have been only once. There were lots of veggies and herbs and an assortment of other things - LOTS of tomatoes. I'll link a page below that tells about their Garden Village and festivals. It's a pretty long drive from the highway to get there.

Jenks Oklahoma (south of Tulsa) has an herb festival in the spring. I haven't been to it but if you ask in the Oklahoma Gardening Forum, you might find someone that's gone in the past and can tell you what it's like.

Here is a link that might be useful: Baker Creek