Online is not a bad way to go. Don't start off too high on the compression because you may not tolerate it. You will need a tape measure.
Jobst or
Juzo are good. I did not search through the website tonight, but they have instructions for how to get the size right. You can often get them in drug stores, or even in department stores.
Don't get on a plane or take a long road trip in the car without wearing support stockings. Pulmonary embolism would be a rough way to go.