Plant the cross-breed and hope you get Poor Muddy Foot Plants so you can get more seeds.
The Sweet Lobelia Seeds are obtained from the Crafting: Seward Proudfoot's First Breakfast quest. You can get 6 if you choose them. That's two cross-breed fields worth. They apparently drop as well, but don't know if that has been proven.
With care and attention, you can multiply your stock of seeds. I started with those 2 first fields and now have enough seeds for 25 fields and have 75 of the Muddy Foot Pipe-weed. Which, when smoked, creates 3 smoke rings as an end effect.