If by “good source” you mean adapters which actually will work without dropping too much voltage at high current, they are hard to find, especially at reasonable price. By “hard to find” I mean that usually sellers and manufacturers do not specify voltage drop per A, and many adapters look similar and do not have exact model number. A lot will depend on your power supply as well, if it has just 5.00V at its output, it probably will not be able to provide 5A through barrel jack even without an adapter in the way. But good power supplies usually provide higher voltage, 5.1-5.25V at least.
I suggest choosing few different Jack-USB-C adapters and buying one piece of each to test. Then when you receive them, start testing them one by one. Load both CPU and GPU to 100% and plug in all USB devices you plan to use (if any). Then run (preferably few times and calculate average value):
sudo cat /sys/bus/i2c/drivers/ina3221x/6-0040/iio:device0/in_voltage0_input
It should never be below 4750 mV. Even if you do not plan to use it under 100% load continuously, it still may happen for short time during normal use and Jetson Nano may throttle or even become unstable if input voltage falls below 4750 mV.
After testing few different models of adapters (on AliExpress, eBay and other similar sites there are many available), it would be possible to choose the one which caused least voltage drop and has the best price. If you need 50 of them, this is especially important, and would allow to make the best choice.