If the goal is to obtain these two supply voltages then it
seems to me you want to rip an ATX power supply out of
any of a bazillion cast-off PCs.
If the goal is to amuse / educate yourself by building a
power supply yourself, that is another matter.
I'm sure you can find many, many designs for supplies
that use the well understood, rather old Unitrode /
Silicon General 35xx (voltage mode) or 38xx (current
mode) PWM controllers. You will be able to find many
SPICE (macro)models for all of them (picking the best
one, may be a challenge) and probably in the Yahoo!
LTSpice group archives, full projects to play with and
modify to suit.
Since it'a s hobby thing I doubt you are concerned
with attaining the highest possible efficiency, smallest
size, highest operating frequency or even lowest BOM
cost (sure, nobody likes to waste money, but for a
one-off it can't be helped really). So I'd recommend
sticking with tried and true (not to say "way crusty")
general purpose parts and all the developed design
knowledge base (TI/Unitrode app notes, for starts).