It might also help to note what software you're rendering in.

For foliage, Poser or DS should be able to import .obj. Thus something such as Arbaro might let you make nice looking trees and bushes to use within it. If you use other apps to render, some of those also have their own built in tree generator. There's also a neat ivy generator program.

Now if you need certain yard furniture elements beyond foliage, it might help to specify. (A rough sketch, or a link to a photo.) There are different styles of lawn furnishings and architecture, so saying you want a gate leaves a rather wide area of interpretation. Some of these things may already exist in on an individual basis in the marketplace, but they require a little arranging in order to make a scene. Other modelers and I also might not mind modeling some various objects, if there is some demand for them.

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Barbequed Pixels?

Your friendly neighborhood Wings3D nut.
Also feel free to browse my freebies at ShareCG.
There might be something worth downloading.