To answer the specific question, there are no open ended tickets.

Your choices are (1) book an inexpensive ticket, pick an arbitrary return date and pay the change fee when you finally confirm the return date; or (2) buy a more expensive but changeable ticket and again, change when you have a confirmed return.

Don't forget that as well as the change fee you will also pay the fare difference between what you paid and what the return portion of the fare is now you have a confirmed date - this will not be a trivial amount.