Comparable property sales is usually a little more complicated than pulling them from a website. The requirements for use in protesting tax valuations are posted at the appraisal district. You should refer to those needs first. Also there is a time limit to file, normally 5/31.
A Realtor can help you. They just need to know what information to give you to help with the process. Typically, the information is not publicly available, though, since it involves the actual sale prices of comps. Texas is a non-disclosure state, meaning that the exact price is not public information.
Realtors almost always have access to the sensitive data, but our code of ethics prevent revealing that data to persons not known to us. In a few circumstances the data has been expunged by the parties and some Realtors do not know how to recognize prices that have been zeroized.
Since the time nearly 6 years ago when you and Robert bought the house, your tax valuation has risen only about $5000, or 3% roughly. A neighbor across the street from you had a whopping 12% increase just last year on a similar size house (comp) and is assessed higher at the moment.