Thanks, but I was referring to the input source, not the output. e.g. charging the single lithium from a wall supply instead of a solar panel. If it expects a certain voltage/amperage from the solar panel, what would be the problem in getting that instead from a wall adapter?
Ebay, buy a handful of these...
search for "Converter Step Down Module LM2596" cheaper than chips! No really, you can't buy the parts for the money.
Obviously you will need your wallwart to be able to supply enough current, so you need to know the C rate (current) that you will charge at (always over specify as wallwarts are usually poorly regulated and over optimistic when quoting current, you don't want another source of fire or failure.
AC wallwarts may be better for you, but remember that after rectifying & smoothing, your voltage will increase by root 2 (about 1.4 times) So bare this in mind when specifying your voltage regulator.