Sacramento’s longtime booming commercial office market has hit a major problem – too much available space and not enough tenants, according to industry reports.  And that empty feeling will likely continue for some time, as developers are building almost 3 million square feet of offices, the most since at least 2000, and some housing-related companies, such as homebuilders and mortgage lenders, are closing or moving into smaller space. That 3 million figure includes two downtown office buildings with a combined 800,000 square feet – enough space for about six Wal-Mart stores. In fact, Sacramento developers are building more office space than St. Louis, Orlando and Fort  Lauderdale, combined. Elk Grove and Rocklin are the hardest-hit cities in the four-county region, with office vacancy rates of more than 20 percent.