Most any airline will sell you an open ticket. You cannot buy them online. You’ll have to buy it from a travel agent or directly from the airline.
You may be interested to know that open tickets are usually very expensive. The cheapest fares require you to confirm your flights and dates. The more flexibility you want, the higher the price goes. I think that you might be very surprised at the cost.
As an alternative, check the cost of buying the lowest fare and paying the change fees to adjust your date later. That will still cost a lot extra, but it might be much less than an open ticket.