For APS-C UWA, your only option for 85mm filters is to use the Cokin Wide angle holder or equivalent. This holder is a single slot holder. Even with this holder, when used on a Tokina 11-16 UWA on a Nikon DX body, I experienced vignetting at 11 - 11.5mm. 12mm is usable but can detect some light fall off.
Funny thing is I have heard people having no problems even at 10mm when using the Sigma 10-20mm lens. This might be due to the construction of the lens.
Canon crop sensor shooters can even worry less, because of the crop factor that Canon adopts is 1.6x instead of the more popular 1.5x. That means Canon users have a little more leeway in vignetting, at the expense of angle of view, of course.
As for filter holder for 100mm, you can go with many options. Lee, Hitech, Cokin-Z-pro, and other China copies. All will work with 100mm filters from all brands except the specialized ones like CPL, or big stopper.