My audit appeal has been denied by the IRS. Can I hire a lawyer for federal tax litigation to fight this appeal?

It sounds like the IRS made unfavorable findings during your tax audit and have subsequently denied your appeal.  Once this stage is reached it is absolutely critical that you get assistance from a tax attorney who can advise you on the next best step.  The only real option is for you to commence federal tax litigation and argue the finding of the IRS in a federal court.  It is important to have an experienced attorney to help you through your federal tax litigation in order to have the best chance of arriving at a favorable outcome.  It should be noted that if a tax preparer gave you tax filing help at any point in time, that tax preparer may not represent you in litigation before the federal court.  You will need an attorney if you wish to proceed after the IRS denied your audit appeal.