I am going to tell you something that you won't want to hear. No one is going to walk into some sort or the other camera place and just pick up a Lee Big Stopper they are always on back order by months. It is the one everyone wants and they are hand made, now it depends on what you want to do in the future as to what you get.
If you just want something with the power of 10 stop then the B+W 77mm filter and a step up ring 67mm>77mm is all you need, it is excellent glass and quality in workmanship and materials.
If you want usefulness then the Singh-Ray vari-NG filter is a great choice you go from 2 stops all the way to 8 stops and again one setup ring 67mm>77mm. Costs more but by the time you work in 6 or so Lee GND filters close in price.
Now if you want to use GND filter then you want the 4"x6" ones so you can position them where you want them. If you went with the Lee system you would need the 67mm and 77mm holder rings, Foundation Holder (holds up to 3 filters), Big-Stopper, and either a 2 or 3 stop 4"x6" GND filter or both then you would have equal to 2, 3, and 5 stops plus the Big-Stopper.
I am going to add a link to a place in the US that handles the Lee and also B+W, as there is almost no weight to the items why not order online and pay the postage. It maybe worth it, to find out it will only cost you a phone call or e-mail to find out.
It is just you can not walk into a camera store and pickup a Big-Stopper off the shelf.
http://www.2filter.com/
Cheers: Allan