If you have decent hand skills, you can make one using a "school" protractor, some straight edges, and a screw and nut. These are very simple devices, and if only for comparative or progress purposes you don't even need that much if you have a friend.

Get a sheet of foam core posterboard. Use string and a tack to strike an arc so you can cut it into a large semicircle. Now, you can use one side up to the center of the arc against the thigh as the reference, and mark off the point where the lower leg ends when flexed to the max.
Add the date and repeat at intervals to measure improvements. If you wish, you can use a protractor to determine the angles, so you have data to share with a therapist, if any.

BTW - we're in the age of digital photography. So if you have a friend/family member who can take photos from a consistent side view position, you can simply take the photos for progress and comparison, and can even measure the angles.