Just create one.
Create a new Automator Service which receives no input in any application.
Add a Run Applescript action with this code:
set newVolume to (output volume of (get volume settings)) + 10
if newVolume > 100 then set newVolume to 100
set volumeoutput volumenewVolume
Save the Service, then give it a shortcut in Keyboard System Preferences. You'll find it in Shortcuts tab, Services category.
Create another with this code to turn the volume down:
set newVolume to (output volume of (get volume settings)) - 10
if newVolume < 0 then set newVolume to 0
set volumeoutput volumenewVolume
For mute, use:
set isMuted to output muted of (get volume settings)
set volumeoutput muted (not isMuted)
The setting runs from 0 to 100, so you could use any increment you want instead of 10.