You need clown makeup. There are many websites available for clown supplies. I've used Jim Howle, but use Ben Nye creme makeup and Mehron pencils now.
www.clownantics.com/makeup.shtml has many brands to choose from. Unless you know a clown who can help, I'd get a good How-to book as well.
Facepaint is something else. Check out "Show us your BEST" under the Face Painting section in the forum. Jitterbug posted some faces there.
You have to know that your parents want what's best for you. They probably think "clown" and envision you running away with the circus, traveling from town to town, cleaning up after the elephants, living in a train, etc. They'd rather see you become a doctor.
Let them view this as a new hobby for you and not an occupation. That way, they may not be so reluctant to help you get supplies.