Here are some links you might find useful:
Giggle Poetry is a website from Meadowbrook Press who also publish books of funny poems.
Kenn Nesbitt is a US children's poet who specialises in funny rhyming poetry.
Fern's Poetry Club is part of the Public Broadcasting System's website.
This site on Poetry Zone is the text of a how-to book by Roger Stevens.
This part of the Scholastic site features writing with Jack Prelutsky and other well-known children's poets.