Limnophila Sessiliflora is classified as a noxious weed by the USDA so you will not find it for sale through your LFS or online retailers. Your best shot at getting some would be from a fellow hobbiest in your local area.
I don't think most nationwide hobbyists would have a problem shipping it to you. The Postal Service would have no reason to open the package, much less take the time to identify the species. As long as you don't release it into the wild, it isn't really a problem.
I have some that I ordered last summer but need more for new tanks. It makes a great filler plant and I love the way it looks in a strong current. I had bought it from someone in Texas but she found it was illegal to sell in that state.:icon_conf As far as I know I was the only hobbyist in this area ( except for one other I gave some to ) that has it. That`s why i`m posting in a plant forum.