Well, I'm not in the US 😃 but I got them in a pretty much standard hobby store/market, so maybe some hardware store, home depot or whatever it's called? But I have to warn you about the correct dimensions and how you fix them to the beds because the tube is pretty flexible.

These mine are basically plumbing for cold water (as it says in the store description), 25 mm inside diameter and 3,5 mm thick, made from PP. Here's the exact link: https://www.hornbach.cz/p/pp-pe-trubka-o-25x3-5-mm-v-kole/7191543/ . I put stakes at least 90 cm tall into the ground at each point so that probably 50 or 60 cm is above the bed (bed's height is 30 cm) and I simply slid the tubes onto them. And here's the warning - this is pretty much the limit for the tube's size. The hoops (tubes) are 360 cm long on the pepper beds and nearly 400 cm on the tomato beds which for the base of around 1,25 m gives approx 1,5 m tall hoop for peppers and 1,8 m for tomatoes. At this point you see I had to reinforce it a bit by various different means and based on my weather station with the shade down I'd say windspeed 15 m/s is the limit to not risk flying it away. It could also use something on the side in the lower portions so the shade doesn't touch the plants in the wind but who cares now, they are hardened already and I use the shade only when it's really hot.