The 2015 model service manual says o-ring drain plug is 11900045. Only thing I heard about the drain plug is that it is easy to strip the threads in the engine cover by over torquing the plug. See several conversations on this forum.

Another thread says they are $0.35 each at parts counter. IF not near a dealer then google says this aftermarket sells them by the 50 pack for $9.
https://www.captainoring.com/store/p202/harley11105orange50pk

 

Thanks I got my dealer to mail some to me . $8.00 & change for 4 ea. but at least I know they will fit , your # is correct .

 

You can get them at auto parts store. Change the O ring every time. Do not over tighten.