For L. sedoides, it's not commonly cultivated for commercial sale so you need to keep your ears close to the ground with collectors, best place for this is (regrettably now) facebook. Regarding aquarium suitability, it's really more happy as a conservatory pond plant. In aquarium situations the difficulty is meeting it's light expectations without also making life difficult for your submersed plants. This can be achievable but is certainly a tricky balancing act.
For alternative surface plants beyond floaters there a few Nymphaea types that can survive in the aquarium and although it doesn't produce a neat surface rosette like L. sedoides, a possible alternative 'show stopper' Ludwigia for consideration is L. sphaerocarpa, which is a big and flashy species with a broad compact rosette of leaves, definitely a focal point plant which can happily live underwater. I believe aquasabi sell it, as well as Alan Yusupov at theaquatank.com.