Not sure I can add more info or better advice than you, really. That fixture hasn't been set yet, so the match could be played on Fri, Sat or Sun. There was an official ticket exchange on the Schalke website, but it has been shut down for unstated reasons this season. Looks like Leverkusen sell their away allocation for local games only to season ticket holders and club members (obvious reason - they never ever sell out their away allocation, if on general sale, all the home fans would quickly register and take them...), keep an eye on the Schalke web site, there might be some seats available nearer to the date of the match, that originally had been allocated to Leverkusen, but been given back to Schalke.
@Cellarite Are you still planning to come over for the Cologne - Leipzig match?