I left a WA company (on good terms, as far as I knew) after 7 years to
try to find something better. Six-nine months later I found out the
ex-employer was sabotaging my job search by wrongly giving me bad
references (either to force me to to return, by making alternative
employment impossible, or just to give me a hard time) (no, I don't
have evidence strong enough to stand up in court).

In my search for some kind of solution, I came across the following AZ
law, Ariz Rev Stat 23-161, which states in part: "It is not unlawful
for a former employer to provide to a requesting employer...
information concerning... job performance to be used for the purpose
of evaluating the person for employment... A copy of any written
communication regarding employment must be sent by the employer
providing the information to the former employee."

So now I'm thinking about moving to AZ, because even though it won't
help with my past employment problems, it might help in the future:
ex-employers are less likely to lie about reference information if
they have to send a copy of any written references to the ex-employee.

My question comes in 2 parts: 1)what other states have a similar law,
and 2)how effective is the law? (For example, do AZ employers
routinely bypass this law by doing all their reference checking over
the phone, never by mail/fax?)

Unless they have locations in AZ, the WA company will not be held to that law.

"... A copy of any written communication regarding employment must be
sent by the employer providing the information to the former
employee."

Many  - most? -  requests and responses will be telephonic.  If the
employer had or has provided written information about you that is
incorrect, that could be libel or defamation of character  - so he
probably has not done so.

I am very sorry to hear about your problem, Myoarin