Here are the IRS instructions for Schedule C. Business codes start on page 19. If you can't find a code that applies, you can enter 999999.
http://www.irs.gov/pub/irs-pdf/i1040sc.pdf
See the FAQ on Business Codes,
https://ttlc.intuit.com/community/business/help/what-is-the-business-code-used-for/00/25672
