No, there is no requirement for Internal Auditors to be certified. They are to be selected, trained and competency of them is to be tracked.
Based on benchmarking with several other companies:
The best way to achieve this is to put a list of auditor traits together ( "what do YOU look for in an auditor" )
If you have a certified Lead auditor on staff, they can train. If not, select a training source. (Don't forget to have them provide their training and competence evidence)
Have the auditors perform audits under guidance of a Lead Auditor. This person can provide evidence of competency.
Ensure they receive training in core tools and CSR. AIAG has a great tool ( free) to evaluate their knowledge of the core tools.
Select a frequency that you can meet, to ensure competency is evaluated on a continuing basis.
Select a minimum number of audits ( typically on an annual basis) that an auditor must perform in order to remain active ( or qualified)