If you buy a properly zoned lot (a lot that will allow a mobile home), and you comply with applicable zoning requirements (for instance, height of fence, setback from the property line, etc.), you shouldn't have a problem. Try Craigslist. I just checked it at http://austin.craigslist.org/rfs/ and there are a number of lots listed in and around Round Rock. Or check with a local Realtor.