Bartering is becoming increasingly popular and it is an excellent way to get supplies if you have things you want to get rid of. You can join a regional bartering group or use a trade or barter forum on a website like Craigslist. Essentially you offer a service you can perform or an object you do not need in exchange for the service or object that you do need. You can also approach people, businesses and groups directly to see if they would trade with you. Perhaps you can convince the local art supply store to give you some supplies by repainting the exterior of their building. Think creatively in terms of what you have to offer.
Get your students to participate in an art show to benefit the classroom's supply closet. Besides inviting parents, invite school board members and the general community. Use the Internet, newspapers, local TV stations and anything else you can think of to market the art show. Charge a modest entrance fee to the show or ask specifically that the entrance fee be paid in the type of supplies you want.
Think creatively. Turn your lack of supplies into a teaching moment and gain your students help at the same time. Instead of focusing on crayons and paints, have your students collect things like cardboard boxes, plastic bags, yogurt cups, packaging, old building supplies and anything else you can think of. Go to "upcycling" websites or craft websites like Instructables to learn new projects to teach your students. This is an excellent way to teach your students about ecology, green living and recycling.
When you go to the manufacturer they become your sponsor. Sponsorship has both pros and con and may not be suitable for everyone. Getting a sponsor is a similar process to soliciting donations except that you focus solely on companies. These companies can donate the supplies themselves or donate money for you to buy them. In exchange for doing this they usually reserve certain rights to your time and image, and you essentially become a walking advertisement for them.
Getting donations can seem difficult if you've never done it before, but it is really not too hard. Make a list of people and businesses that seem promising. Then decide how to recognize their contribution. People want something for their time and effort whether it's a small party or a blurb in the school newspaper. Businesses in particular will love any recognition that may lead to business from parents. Contact the businesses and explain that you are looking for donations and then tell them what they get in return. Sometimes only one out of 100 contacts will make a donation but it is still worth a try.