There are many websites and blogs dedicated to all sorts of information about Revit, from tutorials to downloadable content, though there is very little that is specifically for site/ landscape design.
RevitCity. One of the largest sharing sites, and thus has a large selection. Quality of content can vary widely. Free. Requires login.
Bega. The US website has Revit families of most BEGA lighting products (of those available in North America). Click on product number, “Revit BIM Model” will be listed under Technical Information. Free. Does not require login.
Autodesk Seek. Very limited, though a few site product manufacturers: SiteScapes has site furniture, greenscreen has green wall and screen families, and decent selection of street and roadway lighting. Free. Requires login.
Landscape BIM Blogs (by Revit users)
Landscape BIM. by Henry-Fenby Taylor, landscape architect in the UK
Revit Landscape. by Åge Langedrag, architect/ BIM Innovator in Norway
Online Communities/ Forums
Not all are landscape-specific, but are still good resources for general Revit questions.