Many orchestras will hold open auditions. They'd post them on their website and probably a few regional music target outlets/websites. Also networking, if you

Many orchestras will hold open auditions. They'd post them on their website and probably a few regional music target outlets/websites. Also networking, if you do gigs in the same region the orchestra is located you're bound to bump into a few of the members. You can also just email the orchestra and ask when they hold auditions.