Typical FACS buffers consist of Ca/Mg free PBS with little protein (and sometimes other additives) to improve cell viability, decrease unspecific staining, and avoid sticking to plastic surfaces. Suspending cells in culture media is usually not recommended due to relatively high autofluorescence, poor pH buffering in typical flow conditions and presence of adhesion promoting components.
Start with PBS containing 0.5% BSA (or 1-2% FBS) and optimize as needed