Buy a current copy of "Artist's and Graphic Designer's Market," which compiles numerous market listings for companies looking for poster art. The "Artist's and Graphic Designer's Market" is an invaluable guide for freelance artists because it contains complete submission information and guidelines.
Read through the market guide and select a couple of markets that purchase the kind of artwork you've created. Bookmark those markets that look promising.
Prepare your submission as stated in the submission guidelines. The guidelines will tell you what type of work the publisher purchases, how to format and submit work for consideration, where to send your work, and what you will be paid. Follow guidelines when you submit to decrease the chance of your work not being considered.
There are no specific guidelines for poster art submissions. Individual publishers use individual guidelines, which is why following the guidelines in the market lists is so important.
Submit your artwork to the markets you've selected. Many market listings indicate the response time; don't follow up on your submission until the response time has elapsed. If the listing doesn't indicate a response time, wait two or three months before you send a query.
Continue to submit to other publishers. One of the keys to a successful career in freelance art is to create a cycle where you always have responses coming in always have work under consideration.