You have several options...
1) Hire a full time internal auditor.
2) Fully outsource the internal audit function.
3) If you have a qualified person (thoroughly familiar with audit and internal control) in house, but can't justify the full time position, you might get them to coordinate the internal audit function. Specific activities can be allocated to different individuals in the bank. For example, someone on the lending side could do the audit procedures for deposits. Someone in deposits could do the audit procedures for lending. Etc., etc. As long as the person performing the procedures are qualified and independent, this is a workable solution. Key words...qualified and independent.
4) Similar to number 3...but if you don't have a qualified candidate to coordinate the function, but do have sufficient staff (qualified and independent) to perform all or some of the individual audits, you might get a third party firm to help design the audit program, including audit procedures and checklists. The third party can coordinate the program, with much of the work done by existing staff. The third party could fill in the gaps where you don't have the resources or expertise internally (i.e. IT).
My opinions are just that...my opinions.