I only have one AR that needs one and I use a UTG. It was less than $30 if I remember right.
It's easy on and easy off. Very stable.
Mounts with a picatinny rail or, a sling stud with an included adapter.
There is the harris style as shown in the post above.
Atlas make a nice, more modern, pict rail mount option that I prefer over the harris style. Clones exist and can be found on ebay
Looks like UTG and others are making separate leg modes to mount on either a keymod or mlok rail. Those look interesting, but lack the pan and tilt features of the atlas.
Atlas is not exactly low end, lol, but they’re my choice. The *green blob* clone is decent but needs a little tweaking at the mounting nut to keep it from working loose and to be able to pan freely
It extends and you can change the leg angle several degrees fore or aft
I prefer these due to the difference in size and weight over the spring type, whether I’m using the Atlas ($200-$300) or a 30-60 dollar clone.