In the US we have tons of it, both public and private land in sparsely populated nearly useless areas. Southern CA, AZ, and NM is the best place for solar (desert). Wind is best in about a 500 mile swath going north from the Texas panhandle.
Assuming that solar panels take up 1/3rd of the space where they are installed, power generation would typically be 250,000 kW-hr/km^2/day, or 91M kW-hr/km^2/day. US/capita consumption is ~13,000 kW-hr/yr, so 1km^2 of land would supply 7,000 people. For 325M people we'd need about 46K km^2, or about 215 km x 215 km square, or 1000 plots that are 7 km square. It would be very easy to find that much space. And I probably over estimated how much space they need around the panels.