Smart to get a new appraisal as well as get comparable from a Title Co. or your Realtor who sold you the place
or a Commercial Realtor that you should be in contact with.
The 2006 appraisal won't be your best defense for challenging the 2008 assessment. If I were you the first thing I would do is find out from the Dade County assessor what approach to value they used to determine your 2008 assessment. Chances are they are giving more weight to the income approach to value, but recent sales could also be a consideration. Mott may be right in suggesting that you get a new appraisal but make sure you know three important things from the local assessor first: (1) the effective date of value for the assessment being challenged (2008?), and (2) the timeframe studied for commercial property sales; and (3) the period studies for income and expenses. Your new appraisal will want to uses the same effective date, and consider sales and income and expenses for the same time frames.
Contact a title/escrow company in your area, and ask for their customer service department. Tell them what you need the information for so they can pull comparable sales for the time frames you need. Good Luck!