Is it safe to leave a broth simmering overnight on an electric stove?
Is it safe?  That depends on a lot of factors.  Generally, no.  It isn't.  A blog post from the Healthy Home Economist has the opinion of a firefighter:

One gal mentioned that her husband was a firefighter and that leaving
a stockpot simmering overnight or while they were out of the house was
completely out of the question.

Source.
The NFPA says the same, as this informational PDF sheet illustrates.
Since you cannot get a crock pot my only other suggestion is to simmer your bone broth in the oven.  If your pot is oven safe and you trust your oven's ability to keep a steady low temperature, then try it in there.
If you plan on making bone broth more often, a crock pot is a great investment and if you check out your local Goodwill or Salvation Army store you might find an older but still perfectly usable model for an excellent price.  It would also allow you to slow cook your broth overnight safely (and uses less electricity to do it).