Check craigslist as they are on there all the time used. If you can't find a extension cord, you can always purchase a used regular 50 amp cord and remove the male rv end and replace it with a female 50 amp plug like I did, very easy to do.
2006 Chevrolet Silverado LT 3500 DRW CC LB 4x4 Duramax/Allison LBZ
Reese Titan 20K
2013 Montana High Country 318RE