Free comma worksheets for kids are available at Education.com and HaveFunTeaching.com as of 2015. Education.com provides a worksheet with a mix of different sentences to practice writing and punctuation with a focus on commas.
The worksheet is available as a free download that can be printed. This site also offers similar worksheets for proofreading practice in punctuation and spelling, the proper use of question marks, commas in dates and addresses, and the use of commas in a series.
HaveFunTeaching.com offers a wide variety of worksheets that include the use of commas in a list as well as in general punctuation. The worksheet is downloadable in a PDF file. Site visitors can also access worksheets for nouns, grammar, compound words, capitalization and contractions.