I'm sure there was a campsite at or near Sophia Gardens, but I've just checked and it says there are no campsites in Cardiff at all, so you're probably best to wait for an answer from Grassergirl.
Yes, you will be very lucky to get a hotel/b&b now as most are already booked up, but you never know if you ring around you might just get one. Failing that you'll have to try further afield. I know some people are staying at a caravan for the weekend near Porthcawl and there's also Newport of course. Good luck.