Look at
http://www.formatt.co.uk/ they supply high end filters to the motion picture industry.
As for variable ND filters I bought the FaderND filter advertised around under various brandings and it was a pile of sh!t. I bought it mainly to try the concept and it worked well but I tested filtered and unfiltered and then listed it on ebay immediately (for what I bought it for) because the quality degradation was so bad.
I then ponied up the £s for the singh-ray version at horrific cost (then add $52 shipping to the UK and 17.5% VAT!). I need a variable filter because I use ND to get shallow dof at weddings with flash in bright light, for landscapes etc. where things are not so fast paced and you don't need to change the ND so much I'd go for 3 screw in filters and save a whole pile of cash.
Grads are a whole different story altogether, you need a rectangular filter and a holder to move the grad line. Don't go cheap or you will buy twice.