3 of the 4 burner valves on my 20 yr. old Kenmore (Roper/Whirlpool -
model: 103.7858012) gas range have become stuck in the "off" position.
They did not become progressively more difficult to turn, they just
"froze" one day...(not the same day). The valves are easy to get to
and remove/replace but I have two main problems.
One, I cannot locate a main shutoff valve for the stove, and two, all
of the suppliers (including Sears) that I have contacted say that the
valves are no longer available.

I'm going to take a wild guess that the gas line comes to the stove from the
floor and probably connects to the stove itself via a flex pipe of some
type, correct? If you look at the fitting where the flex pipe meets up with
the feed pipe (typically at floor level) you should find a shut off valve
there. You might need to clean the fitting off with a damp rag or something
and I'll tell ya, it's REAL easy to miss -- I must have looked at ours a
dozen times over and never realized what it was. Chuckle

Can the valves be replaced with an available substitute? If so, where
can it be obtained? Can the existing valves be "rebuilt"? If so, how?
Can I do anything short of replacing the stove which is a double oven
unit in which there is a smaller oven mounted above the 4-burner
cooking surface. Both of the ovens are working properly. Is there any
way I can deal with the stuck burner control valves short of replacing
the entire unit at a cost of $1500-$1800 plus installation, etc. ?

Your unit sounds like the one we just replaced. Ours was also about 20
years old, double oven with a smaller one over top of the burners. We just
said the heck with it and bought a floor model of a single oven unit. We
kind of miss the top oven but it's really not that great of a loss and the
new stove is a much better unit overall. We had considered hitting some of
the second-hand stores that have refurbished/cleaned up used models and the
like but the deal on the new one was too good to pass up.

James