Take 3 PC power supplies and connect 5V outputs in series.
You will get nice 15V, 35-50Amp power supply.
Use 2 or 3 high current diodes to drop the voltage to 13.2-13.9VDC
or change feedback divider ratio in each PSU to drop each of the
outputs to some 4.5VDC for example. That would make for
3*4.5V=13.5V - without use of diodes.
I get used computer PSU for $5 - $10 (Canadian) if working or
for free if repair is needed (still - free parts). I don't think there is
a cheaper way to get so much power (regulated!).
You can also look for Tenma or B+K benchtop PSU for 12VDC,
they cost ca. $300-400 and come with current and voltmeter etc.