Here is an idea for powering your spark core and have power for your sensors etc. I found these solderless breadboard power supplies on ebay (many sellers of them) that plug into the breadboard. They have jumpers that allow you to put 5 volts or 3.3 volts on the power rails of the breadboard. You could have 3.3 on one side and 5 on the other, or the same on each side. It does not matter. It also has an on off switch.
You can power the power supply through a USB connector or through the power input connector. It is the same one/size as on an arduiono. I found a 9 volt clip to 2.1mm DC power plug and a 4 AA battery case that has a 9 volt clip on it. So now I can power my core/breadboard from a 9 volt battery or 4 AA batteries (using the 9 volt clip) , or the USB port.