There are no legit sites for casting calls for professional work, if that's what you mean. At those sites you can only find local auditions for things like student films, indies, theater productions, etc. But mostly, you'll find scams and rip-offs.
That's because it doesn't work the way you seem to assume. Real auditions for professional work are not open to the general public. You need to be invited to auditions. And to be invited to auditions you need to be signed by a good agent (in LA at that point, or NYC close behind). When casting for a new project starts, a casting director is hired. The casting director creates a "character breakdown" of the (specific) roles they're looking to fill for that (specific) project. Then they approach agents they know and trust. The agents go over their client database and submit to audition only the ones they believe may fit those (specific) roles. Out of those, the casting director invites to audition only the ones *they* believe may fit those (specific) roles. The invited actors will then take as many rounds of auditions as needed till all of the roles are filled. That's how it really works. You can't do that through casting sites or ads in the newspaper. Professional productions invest TONS of money and they need to know it's in the hands of people who know what they're doing. No one's in this to make your dreams come true, this is a BUSINESS.
So you'll need an agent. But you can't just sign with an agent. No legit agent takes on beginners or amateurs, only highly-trained and highly-experienced talented people with a strong resume to back that up. That's how you become a professional actor to begin with. So if that's what you wanna do, start from the beginning - build a resume strong enough to impress legit agents. That resume of yours should include training in the form of a top-quality acting school (where well-known and respected acting teachers teach and successful actors graduated from, not just any acting classes), workshops, etc. In addition, plenty of low-level experience in the form of student and indie films, as well as youth and/or community theater (you can find those through those websites, as well as other places like film schools. But be careful! Learn how to recognize scams!). Constantly land leading roles. Win some awards for your acting. In addition, take vocal and dancing lessons as well as other skills you can master and then add to your resume to give it a boost (for example: horseback riding, martial arts, acrobatics, dialects, etc.). It takes about a decade and a lot of money to build that kind of resume.
Before you make a huge investment of money, time, and work, and make big sacrifices in your life, I suggest that you find out if it's even worth it. If that's really what you wanna try and do. Especially if you've never tried real acting. Get into some acting classes, something low-key just to get a sense of it. Find out A)what acting REALLY is, B)if you're even good at it, C)if you're even capable of pursuing it on a professional level (including physically, mentally, and financially!). In addition, learn the business side of acting, learn what's involved and what your job actually is as the product you need to sell. The reality is VERY different from what people usually imagine, and you might be more interested in the fantasy of what you THINK it's like. There's a very good chance it's not for you at all, it's not for most people. If it is, at that point apply to a top-quality acting school (in the big city only) and start working on that resume. Don't even fantasize about professional auditions yet, there's no point. By the time you graduate you will know the answer to your original question and how to go about it. And more.
Once you have a strong resume under your belt, move to LA (or NYC if you're more interested in theater) and continue getting experience and training. At some point, someone you've worked with and impressed (like a director, a producer, an acting teacher, or even a fellow actor) might be willing to refer you to an agent. That's how you usually get an agent. Other ways (like getting spotted at a showcase or cold-querying) are rare.