SRB Griturn they do thier own and supply others!
SRB GRITURN
I use them and
This Shot was taken using a 2 stop soft ND Grad on the sky and no white balance correction needed for colour casts as can be the case with some!
Thier own filters are very good in my opinion and have had good reviews in magazines etc!
You can by a set of ND grads from 1 stop to 4 stops that when using a 3 filter holder can be stacked to get above 4 stops which is only really necessary at sunrise/sunset.
In my experience you usually find the the difference between the Sky and Land on a fairly bright day is 2 stops.If the sun isn't in the shot!
In fact I don't usually meter the scene I just stick in the 2 stop take a shot check histogram,9 times out of 10 it's fine!
Sunsets/sunrises you need more as you are usually including the sun in the shot.
You need the rectangular filters as that way you can slide the to position them with the horizon!
At 24mm you should get away with the P size (85mm wide) without vignetting but you can get a wide angle filter holder to get around this or invest in a 100mm wide set from the off!
As to whether you opt for soft or hard Grads that depends on the horizon!
Also with the rectangular filters you can use full ND's along with ND Grads to extend shutter and balance the sky at the same ie using a 10 stopper!
Hi Tech and Cokin also do lower priced filters but in my opinion the SRB Griturn ones are us good and better value!