I believe that a old iPad charger will work if you have a USB A to USB C cable. It does for me. A better answer would be Apples. 30 watt charger which is a USB C charger. I have read that the iPad negotiates it charging with the various Apple chargers but I don’t know for sure.
My understanding is the max you will get out of the USB-A to USB-C solution is 12 watts, specifically 2.4 Amps x 5 Volts = 12 watts. This would require one of Apple’s previous iPad chargers (some were 10 watts, some were 12, will have to read the charger to see).
The USB-C charger that comes with the new 11” pro appears to be an 18 watt charger, as I got 9 Volts at 1.9 Amps when I measured it, charging with Apple’s USB-C cable.
The USB-C charger that came with my 2016 Macbook is a 30 watt charger, because I get about 15 Volts at just under 2 Amps, again using Apple’s USB-C cable.
The higher the wattage, the faster the device will charge. Based on reading other comments, I believe the 30 watts is the most the 11” iPad Pro will draw, even if you use a higher wattage charger, but I can not verify that for myself as I don’t have a higher wattage Apple USB-C charger.