River Islands at Lathrop sales are picking up steam.Last weekend the four builders each sold two homes apiece. That brings the number of homes sold to date to just over 300.But the big surprise for Susan Dell’Osso, the project manager for the 11,000-home planned community, is who is buying the homes.A third of the purchasers are move-up buyers from Manteca and Lathrop, a third are from Mountain House and Tracy, and the remaining third are Bay Area buyers.“It’s great,” Dell’Osso said. “Not only that it is the local buyers (Manteca, Lathrop, Tracy, and Mountain House) that are buying the more expensive homes while Bay Area buyers are buying the less expensive models.”The expectation was that Bay Area buyers would make up the bulk of the buyers.Currently River Islands sales are leading home sales in the South County.In Manteca, the city’s two leading traditional single family home builders — Raymus Homes and Atherton Homes — have virtually sold out of existing lot inventory and are getting ready to break ground on new lots.