The only way to get more current out than in is with a SMPS (switch mode power supply).
Bob
And you should also make sure that the 24 VDC power source can provide enough current to operate the SMPS that delivers 8 VDC at 8 A. That is 64 watts of output power, so input power capability needs to be about 70 watts (assuming about 90% SMPS efficiency) or about 3 A capability from 24 VDC power source. If more current is available from the 24 VDC source, say 8 to 10 A, then you could use a series linear regulator, but a lot of power would be dissipated (as heat) in the regulator: (24 - 8)(8) = 128 watts. That is why the SMPS solution is better because only about 6 watts is "thrown away" as SMPS inefficiency.
However,
@EFDG you haven't told us what you are trying to DO. Perhaps there is a "better" solution,,, small animals in a rotary cage driving a generator perhaps?