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Does anyone know where I can buy a couple of grey hammerite finished 1950bb size cases?

It's easier and probably cheaper just to buy a couple enclosures from the usual sources and paint them yourself.
You can buy hammerite paint at pretty much any hardware store. Even Wal-Mart carries it. I find the Tremclad kind better than the others. Apply to bare aluminum without primer for best results.
Hammerite is a very easy to apply paint and very forgiving of any defects in the enclosure or rookie painting mistakes. It's easy to get a pro finish with it.

I would paint them myself but I live in a small apartment with no well ventilated space or outside area and a girlfriend who doesn't like paint.