Any idea where I can find a spindle for my 1970 plymouth duster

That's a 9" inch drum brake spindle.
Close.

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It is actually called a "knuckle". The spindle is the part the rotor rides on.
Most people incorrectly refer to the whole thing as a spindle when in truth, the spindle is part of the knuckle.
It is sort of like when people say cement when what they should have said is concrete. Cement is a part of concrete. Taking it further, cement is a component that binds other things together, a glue of sorts.
Have you heard of rubber cement?