Most airlines can sell you an extra seat at a discounted price. You need to call and do this because both seats must be "tethered together" so to speak. It means you often have to pay a booking fee for calling on the phone, even thouigh you cannot do this on-line, but it is important.

When you buy an extra seat for yourself they have to make sure the seats stay together! It is common for people under one PNR to have their seating assignments changed and they are no longer sitting together. That is why it it so important that you call and have the extra seat booked in the right way. No point in having two seats for yourself if they are in different rows.

If you fly US domestic, Southwest has a very simple procudure in place. You buy your extra seat and if the plane is not fully booked they will refund your money. But by buying the extra seat you have gauranteed yourself the space you need.