First, make sure you've checked the 'Geographical Area' for all the programs not only in your state or country, but in those surrounding it. Many programs will travel to multiple states.
If you've done that and you still can't find a program near you, here are some other places to look:
Most private programs and many not-for-profit groups are willing to travel anywhere that can pay for travel expenses. Get together with other schools in your area and have a program come through for a "tour" of your area. With enough schools chipping in, you should have no trouble covering the travel expenses of a long-distance program.
Call any and all science-related museums within 100 or so miles of your location. Ask if they have a science program that visits schools—many do. Try science museums, children's museums, natural history museums, space centers, planetariums, aquariums, and zoos.
Call any and all universities and colleges within 100 or so miles of your location. Even if they don't have a formal program, they may have individual professors or graduate students willing to come to your school. You may have to do a bit of calling around, but don't give up! Start by calling the physics, chemistry, astronomy, and engineering departments.
Big research laboratories often have public outreach programs too. If you have a big research laboratory within 100 or so miles of your area, call them too.
They have a searchable list of US school assembly programs on a variety of subjects (not only science). Note: this site seems to include only private programs (usually fee-based), not university or museum outreach programs.
This is a private (fee-based) program based in Canada, but they have franchises located all across Canada and the US (and other countries!). Follow the "contact us" link on their website to see if they have a franchise near you.
If you come across a program not on our list, please tell us!