My own website has some free skits available for both clowns and puppeteers. There are some other resources available online which I will gradually add to this post.

My website - Truth2tell Puppets and Clowns
Puppetry and clowning website forum where people can exchange ideas, network and find resources.
http://www.truth2tell.co.uk

Clown Ministry
Clown resource website with articles, skits and it's own forum.
http://www.clown-ministry.com/

Drama Share
Lots of free scripts many of which can be used as a basis to write your own clown skits
http://www.dramashare.org/

Clown Act omnibus by Wes Mc Vicar

How to be a Compleat clown by Toby Sanders

Be a Clown can''t remember the author