I hope that your health has stabilized for you and that things are looking up.
While there probably is a water heater and a furnace out there sitting quietly on someone's shelf, gently used, only on Sundays, and kept indoors so that nothing rusted out and started to leak air (furnace) or water (heater), BUT:
we both know how likely that is. They are something that people generally don't change because they want a newer more glitzy one. They change because they have passed their useful service life.
However, the water heater is the least risky one to find and use, since it can only wet your floor (don't ask how I know that
) but a furnace is a bit more tricky: LP gas being used when you're there, possibly sleeping.
has them, might be worth checking...?
They are both pricey items to buy new though, so I wish you the best of luck finding good used ones.