Sigma 12-24mm and Gell Filters

OK, this may seem like a daft question but I know I need a gell filter for the rear slot on the 12-24mm lens but every time I try googling it to try and pickup some site to show me what I need, I get knowhere.:confused: Can anyone point me in the direction of a site that has them so I can work out what I need to get. I'm really only looking for a red filter anyway for B&W.

Thanks

Kodak Wratten filters are high quality. I'm guessing the 3x3" would be more then big enough and you could cut them down to size.

Other then that surf over to the B&H website and browse the filter area. Lee makes good filters. So do others.

Thanks Nick, that would explain why I was getting confused, expecting them to be a specific size available. I shall look about for some.