I definitely agree with method-1 and method-2, but I would like to also propose method-0 and method-3 to be comprehensive:
0 - Build a thevenin equivalent circuit, i.e. just an ideal voltage source and series resistor. I usually construct this by measuring the output unloaded and then once again under a load that fits my operating conditions. Then you can calculate Vt and Rt from this data.
3 - See if IBIS models are available from the manufacturer. You can use a tool to convert them into SPICE subcircuits. I actually just made a post about this: IBIS models within ngspice/KiCad
I typically use method-0 and method-1 the most frequently since they are the easiest to deal with and give very good approximations for my typical applications.