Replacement battery for THS720P

Where can I find a replacement battery for the THS720P? The model is discontinued. If there isn't a battery or AC adapter available, what is next step on how to fix the issue.

The THS720P will work find plugged in with out a battery. +12 DC, 1A,; 2.1 ID, 5.5 OD.
More information in the manuals.

A web search for the battery could show a few places with replacements but I can't recommend any or know anything about them.
Battery was BATTERY PACK; 4.8V, 2.8AH;NICAD, 4 C CELLS IN SERIES, 1.05 X 7.78,