I'm interested about using ND filters with the Canon 8-15mm fisheye, but I'm not finding information about which alternatives are available. Are Format or Lee sw150 compatible with such lens?
If not, is anything available on the market?
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The EF 8-15mm f/4 L Fisheye lens has a gel filter holder in the rear of the lens. Gel, or other very thin media, can be cut to fit this holder. Gel ND filters are not as easy to find as they once were, but are still available from specialty companies such as B&H.
Here's a list of the Canon lenses with a 31mm rear gel filter holder:
Here's a rear view of the EF 17-40mm f/4 L. The rear of the 8-15mm f/4 L is similar.
There are also aftermarket adapters such as the Aurora PowerND CR Rear Mount Filter Kit for Canon EF Lens which fits the above listed lenses and allows use of Aurora rear filters. Here's a review of that system.
For some of the issues about using rear gel filters, please see this answer and the comments to Why would I use a rear gelatin filter over a front filter?
There's also this article: How to use a gel filter on Canon wide-angle lenses
If you are talking about true 180° fisheye lenses, then no, no regular flat filter can be used with such a lens without vignetting and cropping of the field of view, regardless of the filter size (even if it's the 150 mm or 175 mm large filters specifically for wide and ultra-wide angle lenses).
However, Tadashi Filters makes a dome-shaped 3-stop ND filter specifically for 180° fisheye lenses. It looks a bit like a smaller version of fisheye domes used for underwater photography, but doesn't require an expensive camera housing.