Square floor pillows are easy to find, but I strongly favor round ones. They are so much more charming and neater on the floor, and never look crooked like square ones can. After use, stacking them up is convenient because corners don't have to be lined up. Cozy and low-maintenance round floor pillows get a thumbs up.
From a design perspective, round floor pillows work beautifully because they break up hard lines. Most furniture is composed of long lines and right angles (e.g. tables, sofas, etc.), so round floor pillows help balance them out.
Shown above from left to right:
MORE PILLOWS ON APARTMENT THERAPY:
(Images: As credited above.)