Try buying an external power supply preferably one with a barrel jack such as this, be sure that you get one with the proper number of amps that you need. You can caculate the number of amps required by adding up the amperage requirements for all of the devices to get you intend on connecting to it. You may use a slightly lower rated supply if you do not expect to be turning on all of your relays simultaneously.
Now all you would need to do is to buy a barrel connector such as this and be sure that you get the right size, then all you would have to do from there would be to connect it to either a bread board or a PCB and connect all of your devices to it. Be sure to connect the ground from your Pi to the negative terminal on your power supply(this is only needed because you are using sensors and not just relay(s))
Note: be sure to remember that your Pi will no longer be able to turn off the power running to your relays and sensors and that you will have to do this manually either by connecting a switch into your circuit or by disconnecting the power source when not in use.