Is it possible to book a roundtrip ticket and leave the return date open? Some time next year I plan to travel from LAX to Quito and back but I will be in Quito for a few months and do not know what my exact return date will be... however I am flexible on the return date and can confirm it at least 2 weeks in advance when I am in Ecuador.

Is it possible to buy such a ticket through online engines such as sidestep.com or do I have to resort to a travel agent? Are these tickets usually more expensive?