A while back I heard that a simple straight angled cut (so as to remove a triangle of film) worked fine. I tried it with my last batch of bulk-rolled film and so far I've had no problems.
If you really must have a device to cut the film to a curved shape similar to factory-loaded cassettes, there are film-cutting templates available. One is the "Ablon," which is designed to cut long leaders for bottom-loading Leicas, although I expect it'd work to cut shorter leaders, too. There's one on eBay right now, but it's ridiculously expensive ($64.99 starting price, no bids yet). I've spotted a few bulk loaders on eBay recently that include similar devices; for instance:
I know I spotted such a cutter alone just a couple of days ago, but I can't seem to find it now. Perhaps the auction's ended....