You can train employees in your company for Internal Auditor course.
You can refer the clause 4.5.5 of ISO:14001 and OHSAS:18001 , which states that Selection of Auditor shall ensure the impartiality and objectivity.
In your own procedure of Internal Audit , you can include that, any person who is competent to conduct such audit, is your Internal Auditor. Only care to take that, he should not audit his own work/activity and maintain his record of satisfactory training with you.
15th November 2011 From China
6th June 2013 From India, Pune
21st June 2013 From India, Mumbai