Your best bet may be to hire one, perhaps from The Flash Centre, as they are available to fit the Quadra, or you could hire the Profoto version.
At least, hiring one will help you to decide whether to buy one or not, and I say this because a ringflash is one of those lighting tools that can be ideal for a few uses but it's a bit like a fisheye lens - they tend to get bought on a whim, used far too often and then put away and forgotten...
If you do end up buying one, then the options are pretty limited.
There's the kit on our website, this is the original Safari (hence the low price) but with the new Lithium battery. A very good unit but a bit more powerful than many people need.
There's the Quadra (the ringflash head is in fact identical to the Safari one), perfectly good unit but expensive for what it is IMO.
And the Profoto, very well made and a good large size, but very expensive.
The Bowens one, if it is still available, is a blast from the past. It's really too big and heavy to hand hold and was designed really for medium format cameras fitted to camera stands. From memory, it had to be pluggged into a very powerful mains powered flash generator, which again from memory was 3000Ws. They do (or did) also have a ringflash attachment for their studio flash heads, again extremely expensive, big and heavy and not really suited for modern photography IMO.
You may come across and old battery powered one, made by Godox and frankly not very good, some years ago. This was sold under various names including Link Delight, but if you do find one on Ebay you need to make sure that the battery is still OK, which is frankly unlikely.