With a 105C driver of course, i don’t know if its constant current, constant voltage or both, or if there are cheaper ones that would be CE, UL or CSA approved
http://www.digikey.ca/product-detail/en/WSU045-3000/237-1415-ND/3094941
You could, but that seems like a relatively expensive way to do it. It’s ‘constant’ voltage, although I doubt that it’s that accurate. The datasheet shows +/-6.8%, but that’s probably at 3A load. Output voltage may be somewhat higher at very low loads.
If you need 3A why not just use the 5V / 5A one that comes from an old computer peripheral (such as a USB hub or WiFi router)? If you can get by with less then 5V / 2A models are plentiful in the form of USB phone chargers.
With any of those you’ll need to use a driver (such as the Nanjg 105c you mentioned) to regulate the output.
Do you have any thrift stores there? The ones around here have bins of those wall wart ower supplies for $.50/each.