I would like to use a filter on my 15mm fisheye lens but I can only use a gelatine filter, why is this and where can I buy them?

Due to the ultra wide angle shooting capabilities of lenses such as the 15mm, front filter use simply is not possible. It would damage the front element of the lens and cause heavy vignetting on the images. Gelatine filters are available from the following website: http://www.galvoptics.co.uk.