Check out a guy named Eric Sardinas... he's not very well known however, he does have his own signature slide called the "Preachin Pipe". He also has some instructional DVD's. I saw him live at the House of Blues in Chicago and I also talked to him and his rhythm player after the show and he's a cool guy. He's only about 30-32 years old too...
Also... get on google.com and search for "slide guitar lessons" or "slide guitar scales" and you should come up with a bunch of stuff. It's not too hard to find anything on the internet you just have to try.