Hiring firms to do due diligence

[Barry Je]

I've read in various books that one can hire a firm to do the due diligence for you.  Can someone recommend me a firm that does this, and what are the costs typically (ie percentage of purchase price, single dollar amount, etc.)

I'd like to do it myself but would feel 1000 times better knowing I'm doing it in addition to a professional company and see what disparities arise between their analysis and mine.