Crotalaria juncea, known as brown hemp, Indian hemp, Madras hemp, or sunn hemp, is a tropical Asian plant of the legume family (Fabaceae). It is generally considered to have originated in India, hence the name, Indian hemp.
Sunn Hemp (Crotalaria juncea) cover crop can grow to 6 ft. within 60 days. The best place to buy it in Florida is probably Hancock Seed Company: $20 for 5 lbs bag, free shipping (as of 2018): https://hancockseed.com/seed-varieties-241/sunn-hemp-seed-516/
Hancock offers multiple cover seeds, for example, perennial peanut and sesbania: https://hancockseed.com/sesbania-seed-5-lb-cover-crop-seed-930.html
Sunn hemp is a different plant from industrial hemp. Sunn hemp seed is legally available in the U.S. Industrial hemp seed is only available as sterilized seed in the U.S. as of 2018. Industrial hemp has very low THC concentration compared to marijuana plants. Sunn hemp has no THC.
Video from Green Cover Seed: