Try Craigslist. Here's a search for mobile homes near Columbia: http://columbia.craigslist.org/search/rea?query=mobile&m
And here's a search around Florence: http://florencesc.craigslist.org/search/rea?query=mobile&
Those two searches go up to $10,000. But sellers of mobile homes tend to be pretty flexible on price. If someone's asking $10,000, you shouldn't have any problem getting it for $8,000 or less.
Also, one good way to find them is just to drive through mobile home parks. People will post signs in the windows. That's probably really the best way.
Hope that helps.
You might find it the best use of time to work directly with a local SC licensed Realtor.
Here is a link to agents in that area that are on Trulia: