I'm sure you've heard this but you can find turbinado in the supermarket as "Sugar in the Raw". I've used it but it has large granules that don't like to stay mixed with my rubs. When I'm shaking the rub onto the meat, the turbinado seems to seperate out.
Drying the brown sugar in the oven works but you have to spread it out, put it in the oven, crumble and repeat a few times to get it right. In order to crumble it up, I put it in a big ziplock bag and go over it with a rolling pin to crush the chunks. It comes out nice and dry, it won't clump and it's a bit finer due to all the rolling.
Edit: apparently you have heard this before..., from Vinny. How did you do that Vinny? It didn't take me 19 minutes to type out this post.