update
I don't know why but slabs are not sold around here even in plumbing supply places.
Now my plan is to buy a new toilet (Niagara flapperless, because I have a 10" rough in and also because there's a tank sweat problem in the summer since I don't have central air).
And then I think I will try to pour a little second floor as some have suggested here, to make it level around the flange and meet up properly with the bottom of the flange on all sides. I'm thinking of using leveling compound and using a rubber baseboard in a circle as the form. Will leveling compound be strong enough? Will it be removable in the future should someone desire to tile the floor?
If I go to all this effort to make a level floor and the new toilet does not have a flat base I will really go crazy then.
Thanks again for all your suggestions. This is a great list, very responsive!
Julia