E-bay and Amazon are good sources, With Amazon if you have Chrome use the HONEY add on.
Marine stores carry a wide variety of 12 volt fixtures but tend to be pricey.
You can also go to LED stores.. I do suggest LED lighting.. To give you an example.
Years ago I bought a Track Light system.... Well it used 20 watt Halogen "Landscape" type spots.. I eventually replaced 'em with 5 Watt Florcesent which were not quite as bright.. I just replaced one of those which burned out (Been over a decade) with a 4 Watt LED... WOW, Bright.. It is.
I've also converted one of my Thin-Light Fluorescents to LED (Actually 2) and will do the other two on a BOB bases (Burned Out Bulb).
And most of the other fixtures to LED also on a BOB basis.
But I've not changed the fixtures.