Many compounders have XLDPE resins w/ additives and cure included, ready to mold. PolyOne might be one source for this type of material, I know they have Monosil (trademark) and Sioplas (trademark) systems.
PolyOne may have a different name for the process or product. Monosil and Sioplas are original Dow (I believe) designations for these compounds/processes and are still used throughout the industry. You will want to specify a rotomolding grade, and any special properties you require.
Or you can contact a vinyl silane supplier (Wacker, Evonik, etc.) for the raw materials and build your own XLPE compound. This is a bit more technically challenging and probably not a good idea.