A broken DSLR mirror is not a user-replaceable part. The camera should be inspected and repaired by a trained technician, because whatever broke the mirror may also have damaged other internal components.

For a Nikon D3100, parts are generally handled through Nikon or Nikon-authorized repair centers rather than sold directly for DIY repair. If the camera is still under warranty, contact Nikon support or an authorized service center and ask whether the damage is covered. Keep in mind that accidental or impact damage is often not covered under warranty, but only the service center can confirm that after inspection.

Best next step: stop using the camera and arrange professional service.