If you haven't already tried this, get a set of close-up lenses that fit your current lens. These allow the camera lens to focus closer. I'm shooting a Canon EOS 60D, with an 18-135mm lens. I normally use a +1 or +2 closeup lens for shooting models. For really small stuff, I might need a +4. You can get them in sets for a reasonable price on Amazon.
If you decide you need a new lens, you'll have to get a macro lens. Be prepared for sticker shock.