Last time we played at Schalke we couldn't get a ticket off City so I ended up going on german ebay and sourcing some myself.
I got a posters few off here them too. It is normal in Germany for away fans to get tickets in the home end but in the blocks next to/near to the away end which we did.
We didn't wear colours to get in, and just walked into the City end from the next block we were in! No problem at all.
I've looked and there are sellers on german ebay again for this, obviously it's more than face value but better than no ticket.
I don't think you can actually buy them through a UK ebay account so you would have to privately message them first and sort it out direct with the sellers.
At least a dozen pairs are on at the moment and one guy has 4 in the next block for 95E a piece.
Incase anyone is stuck, just my take on it. Nor do I endorse or encourage paying over FV however we had no choice last time and it worked so hey ho.