Trying to find a replacement plug would any of them work on Amazon or do I have to get one that is for the S4?

Thanks

Cool GTX
Your asking about the USB cable for the S4 phone ?
 
Any USB with the correct ends will work. 

I had one of the Samsung factory cables cause a slow charge issue; replace the USB cable and phone went back to charging at regular speed

No the actual plug.
 
This.
 
 

Cool GTX
Your asking about the charger "AC adapter"
 
Watch the Out Put : Voltage & Watts (Amperage) is the same as the Samsung unit --- so you do not damage the battery

You will not damage the battery with a different charger (AS LONG AS VOLTAGE IS THE SAME). Phones are internally regulated for current and as you know, electronics only pull what they need at the time. That's why you can have a 1200W PSU with a 600W system and also why my car can have a 270A alternator when there's minimal draw on my batteries and not blow up (gotta say that 1/0 wire is beautiful lol).
 
 
Example: iPhones could only charge at 5V*1A (5W) for the longest time. An iOS update (11.2) allowed the phones to charge at a higher amperage (5V*1.5A = 7.5W). You could still use the same charger(s) as before but if the charger was rated for a higher amperage throughput, it could now be utilized for that and would charge the phone at the higher amount rather than be limited to only 5W (1A).
 
Only thing he needs to care about is the voltage. He can use any 5V charger he wants to. Amperage only defines how much wattage the phone will see as long as it's regulated to be able to handle it.
 
 
A charger rated for 5V*2.1A (10W - think iPad charger) can be used for a regular iPhone which is only capable of 5V*1.5A. The phone will only charge at 1.5A (7.5W) unless it's an older version of iOS in which case it'd charge at 5V*1A (5W) on the very same charger.