If you can't find what you want, get any clip of a suitable horn and use freeware like Audacity to simply cut-n-paste the clip until it is long enough, then add the silence to it, and tag the mp3 to repeat itself.
Audacity can be a bit of a
learning curve (lots of functions) but sometimes the setup goes simply, and the program can do pretty much anything that professional recording
software will do.
Worst case, find a clip of a horn, I'll run it through Audacity and send it back to you.