Where can I find examples of ADS-B surface (ground) position data?
The best source for that data would be EUROCAE Document ED-102A or RTCA Document DO-260B. These are the Minimal Operational Performance Specifications for ADS-B on 1090MHz. They include all the requirements on the ADS-B transmitter, including test data to demonstrate that the requirements are met. 

However, if you don't have access to those documents, you can try and receive some ADS-B messages from aircraft on the ground using an ADS-B receiver and decode those. If the surface position messages indicate good position integrity (format type code 5, 6, 7 but not 8) and the decoded positions appear on airport paved surfaces consistently than you are sure that you've got it right. 

Note that ADS-B surface decoding is not globally unambiguous; you need to know where approximately the receiver is (as in which N/S,E/W hemisphere) to unambiguously decode the surface position message.



The table below shows some encodings that can be used to verify wether your surface CPR decoder works. 

The first four columns show latitude and longitude (in decimal and 32 bit angular weighted binary hexadecimal notation), the next 6 columns show the CPR data.
\begin{array}{|c|c|c|c|c|c|c|c|c|c|}
 \hline
 \text{Latitude}&amp;&amp;\text{Longitude}&amp;&amp;\text{Lat.}&amp;\text{Lon.}&amp;\text{NL}&amp;\text{Lat.}&amp;\text{Lon.}&amp;\text{NL}\\
(Decimal)&amp;(HEX)&amp;(Decimal)&amp;(HEX)&amp;(HEX)&amp;(HEX)&amp;(Dec.)&amp;(HEX)&amp;(HEX)&amp;(Dec.)\\ 
 \hline
-27.93897726&amp;EC21DD4A&amp;153.00998&amp;6CCE9DE7&amp;0BF7F&amp;03636&amp;53&amp;15E70&amp;0CFC1&amp;53\\
\hline
27.938976&amp;13DE22A7  &amp;45.0000&amp;20000000&amp;14081&amp;10000&amp;53&amp;0A190&amp;00000&amp;53\\
\hline
\end{array}
source: EUROCAE ED-102A , table 2-214 (truncated), page 699