I converted my cable park to a USB-C cable (male to male) and lots of adapers (the short sort you can keep in a box) years ago. Yet i have still some devices with a mini-B female port.
Only thing i found online are cables and everything else. Heck, there's even mini-DP > C and ethernet > C in that form factor.
To clarify, i look for something like this but mini-B instead of micro.
edit: Region is Swiss, Europe.
editedit: thanks guys, found one.
They are not common because it is possible to use them incorrectly and cause a fire.
How? Anything with a mini will be USB 2 at most and USB c defaults to lower power of USB 2 without any handshakes with the device which one with USB mini won't do.
Theoretically you could build a male to male contraption from multiple adapters and a cable. Also you could be providing too much current to a device, however this is specific to the combination of adapter, cable and power supply you use.
You already can as these exist:
letting you plug in any existing USB A to mini cables together to get a male to male device - nothing unsafe about that though. So this is not a very good reason to not allow USB C to mini adapters.
Current is pulled by the device - you cannot supply too much current. Devices take just as much current as they need or as much as the adapter can supply. The only way a device would take more than that is by badly designed or faulty - but that is a problem with the device, if the power supply can supply the power there is no issues on that side.
Also USB C connectors can and do by default operate with USB 2 power - supplying 5V and limiting the current to the USB 2 standards and so any existing charger with USB A or mini connectors on. Thus any USB 2 device will only have access to the power given by the spec. You would require a handshake from newer USB protocols to get access to more voltage/current that some USB C chargers can supply.
There is nothing unsafe about any other this baring faulty devices - but if we worried about faulty devices then we would not allow any electronics devices to exist as any of them could be faulty. USB C to USB mini does not dramatically increase any risk of fire or devices exploding no more so than any device using USB mini or USB C alone.
The real reason is there is likely just not much of a market for them so they are harder to find - but they do exist.
Of course those exist, but that does not make them standard compliant. Same goes for power supplies that don't do handshakes. That's not an issue if you exactly know what you need them for, but they're not meant for the general public because you can use them wrong.