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.
This will NOT cause a fire.
Is it because USB C can be host AND device while Mini USB is only device?
I'm not too sure, my reply was a question. I fail to see how it could cause a fire.
Sorry, clicked reply on the wrong comment :)
I'm not sure that there'd be enough voltage to cause a fire.