How to get a constant current source?

I have been frustrated for months working through an introductory
circuits book, as I want to build many of the circuits, but many of
them include constant current sources. Why is it so easy to find
voltage sources at your local store (i.e., batteries!), but searching
for current sources on the web leads to a complicated bunch of circuit
diagrams?

Here is my naive question: using thevenin-norton equivalent circuits,
couldn't I transform a battery (i.e., voltage source) into the desired
equivalent current source using Vth=InorReq. That is, can I put a
voltage source in series with a resistor (as opposed to its equivalent,
a current source in parallel with the same resistor)? What is the
problem with doing that?

Does anyone know where I could buy a cheap but reliable current source?
Why is this so hard?

.

Relevant Pages

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