Don't even think about using bricks, pieces of wood, etc. for holding your rebar up. They will absorb water, expand, and cause the finished concrete to crack.
The only proper things to use are professional rebar chairs, or what is called a dobe (pronounced "dough-bee") around here. The dobe is a small cube of concrete with tie wire in one face. The wire is wrapped around the rebar and the dobe spaces it off the ground. They are cast in flats, looking sort of like a tin of muffins, and are very inexpensive. They have the same properties as the concrete you are pouring and will not cause it to crack. I have seen 1 1/2 inch and 2" sizes. You will need a larger size for the top layer of bar, and if you can't buy them you can make them, once you have seen the smaller ones.