"watts" = volts x amps x power factor. forget power factor (and a couple other things) vA is a more practical unit for these purposes
to get 15 VA per channel OUTPUT (you get about 85% efficiency) you'll need about 1.5 A per channel (that's your power "input") using 12V as your power source (and 12v is recommended).
So you should get a 12 power supply that'll give you 4-5 A for FULL power CONTINUOUSLY, or get a smaller one and see what performance you actually get.
This is a "class T" amp, so not only do you get a good efficiency at "full output", but it's biggest efficiency factor is that it uses power in proportion to it's actual instantaneous output. When the sound is quiet it only uses a little power, and when it's loud it uses more power. Since you're probably not going to be feeding it a continuous sinewave of sufficient amplitude to drive it to the 15W power output point you'll not be using 6A at 12v continuously either. If (for example) your power supply has moderately sized capacitors to stabilize it's output, that'll give it some ability to handle short demands for power beyond it's continuous ratings. Some "wall warts" (the really small light-weight ones in particular) won't play that game though.
You might actually get by with a single 2a power supply if you add a fairly small 12v battery to your system: you'll have it to help carry the load during peak load, and it'll charge during lower volume periods.