Industrializing Arduino - 24V io? (Electronics)
You mentioned 2 items: the supply voltage to the Arduino and scaling up to 24V output.

If the supply voltage you have available is 24V, then I would suggest you look for a DC-DC converter that scales down to 12V. Or, if you are looking to design your own PCB then perhaps use a 78xx voltage regulator. The 78xx are 3 pin devices and only require 2 caps to operate - I suppose a heat sink will not be needed.

To scale up the outputs to 24V I would suggest one of the following options:

a) Relay board / 
These will take the 5V from your Arduino and close a mechanical switch. If the relays are good quality they will go on for ages. And you can switch loads from 0 to 240V and more, DC or AC.
Plus of the relays are sturdiness and that they are quite forgiving for short current overloads. Minus are that relays are " slow" when compared to semiconductors and they can arce when the contacts open - especially when driving inductive loads and that. Arcing will wear down your contacts and in extreme cases generate EMI that might interfere with your electronics. Arcing can be remediated in AC with a capacitor accross the contacts or in DC with a flyback diode.

If you will make your own PCB don't forget the flyback diode on pins connecting to the relay coils or you will possibly fry the Arduino due to a voltage spike!

Many of these boards are available on the web, so it's a quick solution.

b) Optocouplers / 
Also a good option. Functionally they act as a relay. They are a bit less forgiving to current overloads, so chose them wisely.
They are silent and fast acting. Available for AC or DC and up to 600V.
Minus side is that the tend to be a bit more expensive than relay boards. I'm talking about industrial grade devices and not some el-cheapo web site :-)

c) MOSFETS or Bipolar transistors / 
If you are designing your PCB then this could be an option.
Here you can use a MOSFET or a PNP transistor of the adequate ratings and you would be done. Though maybe considering yur time investment the options above could be less expensive in the end. 
I this case you get complete control of PCB layout which might be a consideration.
If you go this way, don't forget to add a flyback diode to the output of these devices to protect from spikes when switching inductive loads (= motors, solenoids).

A big advantage of Relays and Optocouplers is the galvanic isolation they provide, both from the load to the board as between the loads themselves. 

If you could supply more detailed specs of the loads, environments etc I can help you narrow down your choices.