You didn't say what kind of lens you wanted.. I do have a suggestion, if you go on google and put in the search window "Camera lens for rent".
Some of these places do rent Tamron and Sigma lens. I do this when I am not sure what kind of lens I want or if the Tamaron or Sigma would work for what I want to use it for. Lens are big investment and renting one and trying it out is a great way to play with it before putting the money out. All that being said I tend to stay with Nikon lens for my D90 but I do have a Tamron 28-300 lens that works just great.
Good luck on your search...
rent camera lenses - Google Search