If I had tax issues, I would use either a CPA who specializes in tax or an an Enrolled Agent. If you don't know what an Enrolled Agent is go to naea.org --they have a good definition. Unenrolled practitioners (anyone other than a CPA, EA, or attorney) are extremely limited in what they can do for you regarding representation with the IRS.
If you are in a situation where you owe the IRS money, any person who decides to represent you is going to charge an upfront retainer to make sure they get paid. They'll also provide you with an engagement letter.
I'm not familiar with either of those organizations.