It is not trivial to do what you are looking for without buying some kind of adapter. Apart from the IBM products you mentioned, Attunity, Neon, Labrados and iWay come to mind.
If you want to write your own,then in a nutshell, you would need to write a TCPIP socket listeners that would receive incoming requests, then you would have to access the IMS transactions using APPC or OTMA and finally you would have to transform the transaction output into WSDL/XML

Mike