shop-et-al, that's how you do it. Email a scheduler and see what they can do.
When I'm doing a route, i'm usually only booking a week in advance but that's because my personal life is chaotic so it's hard for me to plan any further out than that. I try to get an "anchor" shop or shops -- something that pays enough to cover the gas so at least if I can't get any other shops I'm not out of pocket. Once the anchor is set up, and I know its time constraints, then I apply for shops on the way to make a decent day's work.
Usually I'm working for four to six different companies on a 10 shop route. Oh, and I start out, "I'm trying to set up a route to xxxxxxx, which is 120 miles from me. You've got a couple shops in that area. Could you pay $XX in bonus to justify the trip?" and don't be afraid to get several times your gas needs in total. If one company will pay $40 for the trip, and two more will also, there's nothing wrong with keeping that information to yourself and taking the $120 for a trip that might only cost you $40.
If I have other companies covering the travel cost, I might pick up a shop that helps out a scheduler who doesn't have bonus money. The haircut shops never have bonus money but if i'm going by one anyway and need the haircut I'll pick it up.
Time to build a bigger bridge.