Suppling 5v, 0.02 A (20 mA)
Your circuit will draw what it needs, apparently up to 20mA. So any 5V supply will meet your stated requirement, provided it's capable of 20mA or more (see below). 

20mA is a rather small current, so any decent USB charger will work, as will a power bank (with the advantage that you don't need to plug it in if it's charged). You just need to find a way to connect it. 

If you have a sensitive analog circuit, there may be other requirements for noise, ripple, regulation, for it to work optimally, but for a typical microcontroller digital circuit, pretty much any such circuit will work.

Use a good quality supply with genuine safety markings, there are some really bad USB chargers out there that are actually dangerous. A spare power supply from an old brand-name router or switch would be a good possibility, and probably free.