for more information about WAP Server /Gateway. About free WAP GW availability I do not unfortunately have information. At least you can evaluate Nokia Activ Server 2.1, you can download it from http://www.forum.nokia.com.
one more thought .. there is no such thing like "a WAP server"
Nokia has a product that they call "WAP Server" but actually is bundle with a WAP gateway and a HTTP server (for content providing).
the gateway takes care of the gateway stuff and the web server provides the content.
The correct term for what you want is a WAP gateway.