You won't find it externally. To access the shift solenoids, you'll need to remove the transmission (a major job requiring specialized tools and knowledge) or, in some cases, potentially access it through the transmission pan after dropping the pan. Even then, you'll likely be dealing with multiple solenoids, not just one.
If you're having transmission problems, don't attempt to troubleshoot this yourself unless you have significant mechanical experience with automatic transmissions. Diagnosing and replacing solenoids is best left to a qualified transmission mechanic or repair shop. They have the tools, experience, and knowledge to properly diagnose the issue and make the necessary repairs. Improperly working on the transmission can cause significant damage.
