

I began building up an old steel mtb/dirt jumping frame I had lying around to more comfortably pedal with our baby seat (& baby) on the back

Currently the baby seat is on the back of an old SS road frame - this is ok on paved, flat paths, but not much else!
When I realised that the 3 threaded holes on the frame needed this type of v-brake mount, I came grinding to a halt. I've spent a fair bit of time on Google & have figured out what I need (i.e. the mounts in the photos above), but I can't find them for sale anywhere! If they can still be found for purchase somewhere these days, I'd love to know where, or what term I should be searching with to find them!