While I can't find one off-hand, this feels very much like other accommodation questions. You're after last minute accommodation with a destination and number of people.
I'd recommend trying Expedia, AirBnb, and consider booking.com and hostelbookers (they do sometimes have guesthouses or private rooms - it won't necessarily be in a dorm).
You could also be pushy and ask the accommodation that cancelled to please find you a suitable replacement, as it was their error. Or if you aren't comfortable being that pushy, consider asking them to suggest a place, or if they can call some places on your behalf that they know may have space.
You could also rent a caravan, or go camping - more likely to get a spot that way and don't need a hotel.