You will need a 5V supply on the servo rail to run servos in any case. The most common way of doing this is to use the BEC included in an ESC. If your ESCs do not have built-in BECs then you may use a stand-alone BEC which are available from most hobbyist suppliers.
Regarding setting up buttons to control servos, this functionality is in the early stages and should be included in the next stable ArduSub firmware release. See more discussion about this here.
-Jacob