i found mine in the plumbing section at the hardware store: i believe they're 1/2" diameter brass pipe (called a nipple). i used a long screw (#8 x 3 in. philips bugle-head coarse thread wood screw, IIRC) to remount the strap button and since i use schaller strap locks: i had to grind down (reduce) the diameter of the screw head to accommodate the strap button. i spray-painted the brass nipples (the extensions) black per my own aesthetic. the long screw goes through the button, then the nipple, and then into the upper horn, of course.
TL;DR: i'm not aware of any commercial versions --- i think it's a DIY project!
you can sort of make one out in these pics if you look real closely for it:
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i can't remember which TB member turned me on to this...might have been @Ric5.