Oh, I know a few companies that make them, the problem is finding them without having to order online. Aside from Slime tubes, which are insanely heavy, nobody seems to sell any tubes in shops with removable cores at an LBS.

Maybe some of the super light latex ones, but if you're shelling out that kind of money, you're probably not going to want the extra weight of sealant in them. You probably also have the $$ for a proper tubeless setup.

[edit] And to answer your other question, I have run Stans in a tube, and I have used Slime tubes. The slime tubes kinda suck, road feel is crap. The only bike I use them on anymore is my old Trek that I use to chase my kids around the park on. It has 28's on it with tread, and I run them at like 60-70psi. They are thick, heavy, etc. Not something you want on a proper road bike.

Running sealant inside normal tubes is kinda a PITA with a road bike. First, getting the sealant in the tube is kinda messy, and second, they don't have the same feel as running proper tubeless and kinda ruin the feel of even a good road clincher. I mean, it feels like a clincher, but with 2oz of liquid in it. I'd honestly opt for a proper tubeless setup. I tried it with Vittoria Open Corsa CXs, and it just kinda ruined the feel of them. I love the Open Corsa as a clincher, and while I haven't found a tubeless that has quite the same road feel, most of them are pretty darn close.