I want to buy one for checking knee flexion but they are expensive. The extendable ones are $60-on up. I saw one by Bikefit.com that is clear plastic with ranges marked for $45. Any other options? Do you guys use either of these?

I downloaded several free goniometer apps for my iPhone 6 Saturday morning, and found "Goniometer Records" the best one to use. It, combined with a straight edge, gave me satisfactory 27 degrees.

You can find them much cheaper on amazon https://www.amazon.com/Plastic-Gonio.../dp/B000N549S6. The price is $7. While not extendable a straight edge can extend it to the hip and ankle bones.

I have the Bike Fit branded one. The first one came cracked in the mail but they quickly mailed out a replacement and I haven't had any problems with it since. It does have markings to also measure stem and handlebars. The hardest part for me is finding someone around the house to measure me when I want to double check my fit. It probably only gets used a couple times a year but I am happy with it. The markings are very easy to read. It also comes with an instruction manual on the different angles to measure but I know that info is readily available online too.

Have a friend or use a tripod to take a photo or video of you from the side.

Then use a free protractor app to measure the angle on your phone.

I like video, since it captures actual movement. Then freeze the screen and measure. In contrast, a Goniometer requires you to hold a fake position for measurement.

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.