The 5 and 6 ft. flails are Ford models I picked up at sales. The 7' is an Alamo, I think. I bought the 7' from a Corp of Engineers auction. Older but not beat up, clean and with good flails, also has the trail wheels behind it so makes for easy cutting depth control.
Some folks go out and purchase a 3-5K flail. If you need an offset flail that may not find a use one, so have to buy new. When it comes to cutting, I guarantee my 7' will preform with the best of them. I paid $600 for it! I found all the mowers on Craig's List. Keep an eye out, you'll find a bargain.