lost power supply for a 21 led lamp
The simplest thing to do would be to temporarily use a large 6V power supply &mdash; one that can supply several amps &mdash; and measure the current that the lamp draws. Borrow the use of such a supply and a multimeter if you don't have them. If you don't know people with such devices, check your local area for "maker spaces", which almost certainly will.

Then, select a final permanent supply for the lamp that can provide at least the measured amount of current. It doesn't matter if this power supply can provide more current than needed; the lamp will only consume what it needs.