It's not just mud that is the problem with sand other than pure quartz (silica). Other minerals will leach into the substrate, and simple washing will not remove other mineral inclusions. Another source may be construction companies dealing in sandblasting supplies. Very often this is silica sand, which is uniformly white, like sugar. You may need to ask for "pool filter sand" at the businesses dealing with pool supplies. Very often it is not stocked on the shelves, although I am sure it must be used. Failing this, it would probably be better to use perlite/peat or pure peat vs. sand with other mineral components as inclusions. One thing CP hate is lime, and carbonates are frequently a component of non-silica sand.