Don't know if you need it bad enough to spend $29, but TurboSquid has a 3D tennis court with net that comes in OBJ format which will import into Poser:

http://www.turbosquid.com/FullPreview/Index.cfm/ID/242084

If you want to keep the price to free you can e-mail me and I have a court without a net I whipped up. You could make a net easily by using the plane primitive in poser and simply map a net with a transparency map onto it. You can get good tennis net textures by searching for "tennis net" using either Google or Yahoo image search.

If you want it write me at vislaw@yahoo.com.