I got given a posh mamas and papas changing table from my sister , but she kept the mat, so I had the same problem! So mamas and papas must do them, just a bit too expensive for me.
I found a small (value range) ginham check blue one in toys r us which fitted perfectly on length , but was slimmer !! so slightly gappy. I now use a normal size one and the end hangs over the bar at the feet end, but my little man kicks one of his feet over and over again, so it protects it lots so happy with that.
Hope you find one