If you live in a right to work state, they cannot force you to join the union in order to work. If you do not live in a right to work state, unfortunately the larger productions generally WILL have IATSE agreements and you would need to be a member in order to work. The Catch-22 is that in order to be a member of most locals you need to have verified experience. So the only way to get that experience is by working on smaller, independent productions.
That being said, the movie business is very much a "who you know" not "what you know" business. Keep in touch with the best boy - usually that's who puts together the crew. If you did a good job and were good to work with they will call you again the next time they have a job that you can work without IATSE membership. If you get a reputation for being a good worker, then you may start to get calls from people you don't know. If they are putting together a crew and calls someone they know and they are not available, they can say, "but call this guy he's really good".
Also, if you state or city has a film office, they may produce a directory of "locals" that potential filmmakers can look in to find appropriate crew. Most just require a crew sheet with your name on it to add you to the database.
Hope this helps. Good luck.
I worked in production.