There are plenty of good commercial planting mixes now on the market at local garden centers (not flower potting mix) or you can mix your own. Here is a recipe from Julius Nuccio right around the corner from you at Nuccio's Nursery in Pasadena:

50% sandy loam
35% coarse peat moss
15% perlite
This makes for good drainage and an acid condition. Crushed fir bark can be used in place of peat moss, or in combination.

Also listed below is the website for the American Camellia Society, which has a bulletin board that contains a very interesting discussion string by some commercial growers about potting mixes. You may also find information on the "Camellia Q&A" page under Growing Camellias. Check it out!

Here is a link that might be useful: American Camellia Society