Well, I use a Canon 17-40mm L lens and for filtration I use Lee Filters. The filter thread on the front is 77mm
If you go for Lee then I suggest that you go for the wide angle adaptor over the regular one. What's the difference? That wide version is made so that the filter sits closer to the lens element. By doing this you are less likely to see the line of the filter in front of your lens.
According to Lee's website, they are experiencing a high demand right now. But if you shop around then there are retailers that have them in stock.
If you're still using digital, then I'd just go for the Neutral Density grads of the hard variety.