There are a ton of assets available from a ton of places out there but good skp formatted assets are hard to find, the 3D warehouse does not normally have good enough quality for my needs. So it's normally a case of finding what I need , downloading and converting. Skimp and Transmutr work great when the downloaded asset is an obj or fbx but what if it's .max, .c4d?
There is a service many folk need, converting from these formats to skp IMO.