What SLR/DSLR Lens is this? And where can I find an adapter for it to work on Nikon D3100?

I've recently acquired a lens and have no idea what model, make or lens it is.
All I know is the follow, the case (not sure whether from manufacturer of lens or not) says "Sun Lens" and the lens cap has a uppercase "K" in the center. Here is a link to the image of the lens http://www.imagebam.com/image/6cf2ae190077344

NO

Just look at the mounting end of your existing lens and compare it to the lens in your link. You will see that they are not at all a like

In the future, just look at Nikkor AF-S lenses

You have an excellent camera, do not try to go cheap now.

If you're talking about the rear lens cap, then the K indicates that it's Pentax fit.

Sun was a generic lens - Sun just put their labels on that week's batch.

Without focal length or aperture information, you haven't given us much to go on, but it's a cheap, poor-quality lens and, while you can get a Nikon/Pentax K adapter (try Amazon), you'll lose both autofocus and metering.

It's not worth the hassle.