Where I can find jquery.param implementation code?

/src/ajax.js

Here is the source directly from jQuery (MIT license):

jQuery.param = function( a, traditional ) {
    var prefix,
        s = [],
        add = function( key, value ) {
            // If value is a function, invoke it and return its value
            value = jQuery.isFunction( value ) ? value() : ( value == null ? "" : value );
            s[ s.length ] = encodeURIComponent( key ) + "=" + encodeURIComponent( value );
        };

    // Set traditional to true for jQuery <= 1.3.2 behavior.
    if ( traditional === undefined ) {
        traditional = jQuery.ajaxSettings && jQuery.ajaxSettings.traditional;
    }

    // If an array was passed in, assume that it is an array of form elements.
    if ( jQuery.isArray( a ) || ( a.jquery && !jQuery.isPlainObject( a ) ) ) {
        // Serialize the form elements
        jQuery.each( a, function() {
            add( this.name, this.value );
        });

    } else {
        // If traditional, encode the "old" way (the way 1.3.2 or older
        // did it), otherwise encode params recursively.
        for ( prefix in a ) {
            buildParams( prefix, a[ prefix ], traditional, add );
        }
    }

    // Return the resulting serialization
    return s.join( "&" ).replace( r20, "+" );
};

In case anyone else stumbles across this, it's in src/serialize.js now https://github.com/jquery/jquery/blob/master/src/serialize.js