Every (proper) travel agent should be able to book you a ticket that can be canceled free of charge, which is what travellers in your situation normally do. Note that these tickets are much more expensive than what you would normally buy.
In comparison to the IT systems that travel agents use, the web interfaces of online travel agencies and airlines are rather limited, and as fully flexible tickets are rarely used (except by business travellers, who are likely to contact their favourite travel agency for them any), such interfaces often do not support them. So you may want to visit a real travel agent for that. Some airlines, such as Lufthansa, however support booking fully refundable flights on their websites.
To give you an idea of the ticket price, head over to http://matrix.itasoftware.com and input the data for your wanted flight. Then click on "advanced routing codes" and type "/f bc=y" into both text fields. This will instruct the program to only look for flights in the booking class Y, which can be cancelled free of charge for most airlines. Then search for your flight. After you found it, check the fare rules to confirm that the ticket can be cancelled free of charge. You should be able to print out the fare information and get the ticket booked in any proper agency (for a fee).
