A Common problem that we run into here at Digi-Key is when someone is looking for wall mount power supply they find one with the right specifications, however it does not have the correct size barrel plug on the end. With 2.1 and 2.5 MM being the most common inner diameters we have two common needed adapters.
271-2628-ND is a 2.5 MM ID to 2.1 MM ID adapter. 5.5 MM OD on both ends.
1528-1731-ND is a 2.1 MM ID to 2.5 MM ID adapter. 5.5 MM OD on both ends.
There is also some options for going from bare wire to barrel connector and barrel connectors to bare wire through terminal block adapters.
Digi-Key also has several other varieties of adapters in the Between Series Adapters.