I had a similar problem. What I ended up doing was buying a kennel/cage for the cat and keeping him in there when the house was being shown. It actually worked well.
Other than that, do you have a friend or relative willing to take the cat?
The only other option is to board the cat at a facility. It will be expensive, though, and very stressful for the cat.
Best of luck! Again, I found that the cage situation worked well. But it worked.