Hyundai Repair: hyundai accent 2003, brake master cylinder, hyundai accent

Question
Trying to find where the gear position sensor or a pulse generator sensor is on a 2003 Hyundai accent.

Car seems fine. It shifts into 4th gear just fine several times and the O/D button works fine, so it must be a sensor that needs replaced. Just can't find it  

Answer
The pulse generators mount in the transmission case and are serviced together.  One is located on the top rear of the transmission just about directly underneath the brake master cylinder and the other is located in the front bottom of the transmission case end cover.

The range switch is located underneath the shift lever on top of the transmission.  As you're looking under the hood of the car, it'll be just to the left of and beneath the battery.

Of course, this begs the question:  why do you believe there is a problem?  There is no mention of any symptom in the question.  I ask this because it's generally not cost-effective to guess at sensors without some evidence at least indicating that a particular sensor may be the problem.  Furthermore, in most (but not all) cases, a diagnostic trouble code is stored if a sensor is needed.

Is your transmission behaving abnormally?  If so, how?

Has your check engine lamp been coming on?  If so, your best avenue for diagnosis should start with interrogating the engine control module and transmission control module so you know what the codes are.