Fog machines can now be bought VERY inexpensively. I would check ebay, craigslist and even your local freecycle websites. (I just saw one used for 15 bucks). No matter if you buy or build one, look online for building a "fog chiller" for your fog machine. I'm not sure what effect you're going for, but a fog chiller keeps the fog low on the ground. Might help keep your actors from running into each other.