Screenwriting : Dialogue specialists? by David Copper

I'm wondering if anyone can provide any advice on slipping into the industry as a dialogue re-write specialist. I've already done this with small indie filmmakers, but I'd like to leap to the next level. Is there a special way in the door for dialogue writers or do you pretty much have to get in the way other screenwriters do (spec script, nepotism, tons of luck). Any help would be great!