I am trying to find a website that offers the free listings of
preferred stocks. So far all I've found were preferred stock
newsletter subscriptions.

Dear Adagio, 

Quantum Online lists preferred stocks, online, and has free
registration. It is described as "the best and most comprehensive web
site on preferred stocks, exchange-traded income and debt securities,
and convertible securities. Has outstanding easy-to-understand
information on all types of income securities plus a huge database of
every income stock and security traded on any U.S. exchange along with
income tables, and dividend payment schedules. A truly outstanding web
site."

Quantum Online 
<http://www.quantumonline.com/> 

Other services are not for free, or as you said, offer newsletters. 

I hope this answers your question. Please contact me if you need any
further clarification on this answer before you rate it. My search
terms:  "preferred stocks", related:www.quantumonline.com/,
http://www.quantumonline.com/, "preferred stocks" listing