I don't want my money back, I just want my stuff...
Actually it's not a breach of contract, because they force you to accept their terms and conditions when ordering, and in their terms and condition is this:
"The firm Musikhaus Thomann e.K. reserves the right to deny the promised service subsequent to determining that the goods are no longer available, both 1) after contractual settlement and 2) although a corresponding hedging transaction has been concluded. In such a case, the customer will be notified immediately. All reciprocations and considerations of any kind provided by the customer beforehand will be reimbursed without delay. It is then excluded that futher legal claims be raised by the customer against the firm Musikhaus Thomann e.K.."