"Hrmph! All I have is a power supply with a 2.1mm plug and my doodly-whatsit has a 2.5mm jack. I keep trying to force the plug to fit but that 0.4mm difference just wont work....what am I going to do!?"
ENTER The Adapter. This 2.1mm to 2.5mm adapter is the best way to adapt a 2.1mm plug so that it will fit into a 2.5mm jack.
Remember that this will fit into a device that normally would require a 5.5 OD / 2.5mm ID plug, and lets you use a 5.5/2.1mm plug. Once this is in place you can just use any and all of Adafruit's DC barrel-plug wall adapters, as they all have 2.1mm plugs.