My source was gentleman in Oregon named Kevin who runs some kind of parts store,
kvn.wt.7629@gmail.com - I found his contact on another RV forum while googling the original part. Very good to deal with.
I should add the original cutout for the Sierra light is a bit bigger, so the screws on the new light have *just* enough of an edge to go through, so it's important to center it - worked out on mine, together with some sealant for good measure.
Cheers - Martin