The op is probably coming across my embarrassingly crazy posts from back when my doctor was prescribing me stimulants. [scared]
Anyway, Workshop Heaven used to sell the whole BTI systainer range, including those pouches. May or may not be worth reaching out. I recall they were the only dealer shipping those to the US at the time. And yes, those inserts are exclusive to BTI. That said, working with Bott, Festool/Tanos seem to have caught up with Milwuakee Packout on practicality. Before hunting down expensive inserts, I'd take a look and see if the new systainer3 has something to offer that will fit your needs. I inevitably did not purchase the BTI inserts, but I've gone through all of the other elastic/pouch inserts over the years and I'm not a fan. It's just not user friendly. I've owned B&W style "tool attaches". Very similar. Look great on paper; horrible in use.