Michael's answer is a good one, and probably the way to go for a one off. When reading it though, a similar idea came to mind to maybe consider if you want something a little more "modular", the downside is a little more work to implement:
Mount the relay on a small piece of stripboard, and use a couple of 2.54mm pitch screw terminals for the power connections to the relay (or another detachable method, or solder wires in directly if you have nothing available)
Use a couple of 2.54mm pitch headers soldered to either (under)side of the stripboard to provide connections for the relay coil and support for plugging into the breadboard (the rest of pins can be redundant, just used for support - remember to cut any tracks connected to the relay power pins/header pins to isolate from breadboard. You may also want to add some solder/wire to the traces just to make sure they can handle the current)
Hopefully the above makes some sense.
This way you can plug the relay "module" into the breadboard and attach/detach the stuff you want to turn on/off easily.
Buy a ready made module, but where's the fun in that... :-)