Posting tax return just means that you have filed with IRS but it's possible that your books are a mess or more specifically you are cooking the books.
The responsibility of an auditor is to make sure that your records are complete and accurate, they follow GAAP and statements based on accounts represent company's financial position (Paraphrased from Business Dictionary). The auditor no matter how creative you get has professional and legal responsibility which if neglected leads to disasters like Arthur Andersen. So having all that in mind.
I would look at firms that can perform audits in your area. E&Y, KPMG, D&T are not the only games in town and if you have a 501(c)3 tax status may be approaching them to provide their services in exchange for tax deduction for the service rendered (if legal, you might have to check with accountant or a lawyer).
Another creative way to have an external audit is to request IRS to audit you but I don't know if you will be pleased with results but they would certainly do it for free.