I believe I may have solved this issue. I have found a US-based supplier (located in Saline MI if any of you folks are nearby!) that appears to have these clips in stock as brand new non-genuine parts. I’ve checked with a ruler on mine as best I can and all their listed dimensions do appear to be spot on!
Please see photos below of one of my (already broken) lower trim clips that I removed from my ‘spares’ car this morning:
And here is a photo of what I believe may be a direct replacement plus dimensions:
Suzuki 09409-08320 Trim Board Retainer 20.5mm Total Length, Auveco #19257
And finally, please find a link here to their website and the part plus pricing: https://www.fastener.zone/collections/s ... 9409-08320
(It looks as though they’ve got pretty much every other fastener for our cars as well…