Hornwort is one of the most common floating plants sold at chain stores and fish stores, but they usually have it weighted down in their plant tanks.
Most stores have a really limited selection of plants, so online is a good alternative. Forums like this one (buy/sell/trade section), theplantedtank.net and Aquabid.com have all been good sources for me. It's mostly hobbyists selling, so you can just get the plants you want, and shipping is usually lower than the online stores.