A great resource for plaster art lessons, or sculpture lessons in general, is the art department of your local college. Look for courses that apply to your specific skill level and interests, keeping in mind that more advanced courses might require a portfolio of your work for entry. If you're solely interested in taking a few classes on plaster art, and not enrolling in school, contact professors about auditing classes and purchasing your own materials separately from the rest of the class.
Local community art spaces are another resource for free and inexpensive art lessons on just about any subject. Many community art spaces offer master classes with artists in their area of expertise, so depending on your artistic community, you might be able to find lessons on plaster art being offered. Check the website and bulletin boards of your community art spaces to get information about upcoming classes, and join their mailing list to find out about possible plaster art lessons in the future.
The internet is a valuable resource when it comes to learning new skills and improving old ones. You can find step-by-step plaster art tutorials, as well as lessons on the basics of working with plaster for sculpture. Be sure to follow all instructions when using an online resources, and pay attention to the type of plaster used for different projects. You can also look up directories of local art resources online, such as at Find Your Art School.
Check out books and DVDs on working with plaster from your library, and become a self-taught plaster artist. Go to the "Art" section of your library, and search for "How To" or "DIY" books and DVDs that cover the basics of working with plaster artistically. Art plaster and sculpting tools are readily available at most craft stores, so there's nothing stopping you from experimenting with plaster art in your own home.