I definitely endorse the idea of getting a trusted REALTOR to put together the comparable sales information. Just be sure he uses sales from the year before. For example, the appraisal for 2009 is supposed to be good on January 1 of 2009. If you contest that appraisal, use comparable sales from 2008. Don't use any 2009 sales. The appraisers/review board will throw them out because they happened after January 1.
There are some companies that do this but the fee is very high.
Everyone is correct in having a Realtor do this for you. No the Tax appraiser does not have all this information. They do not search the MLS, there are homes sold that are not on the MLS and they have to figure all taxes the same. This is a non disclosure state and only on the MLS does it show the sales price, the information the tax appraisal office receives is on the loan amount.