If you are trying to stick with a 12VDC light fixture, there are literally thousands available**. As far as a source - pull that existing overhead fixture out, add a fancy flange and install a hanging light by tying into the 12VDC wiring that supplied the light you pulled out. If that wiring supplies other fixtures, install the new one in parallel and either find a new one with a switch built in or install one in the plus feed wire to the new fixture. Not a big deal.
**https://www.rvupgradestore.com/RV-Interior-Lights-s/80.htm or here http://www.hannarv.com/C/LightFixtures/6259 or here http://www.itc-rv.com/ and even AMAZON