The power supply must be regulated. If your pack really puts out 5v to 5.5v, it is regulated.

Just be sure the supply doesn't put out >5.5V under any circumstance. The unregulated versions have a voltage that varies according to wall voltage and load. Don't use one of these.

If the supply is 4.5 - 5.5v under all circumstances though, it should be fine. The load is very light but the supply should be capable of 25ma minimum. I've never seen anything go below 100ma. In other words, practically speaking, you can use the smallest regulated supply you can find.

Something like the Mouser 552-PSA-A05A-050-R (5.15V 1A, 100mv ripple, 5% regulation) at $7.98 in single quantity should be fine.

DON'T FORGET TO DISCONNECT THE +5V FROM YOUR RECEIVER THOUGH!!!!