Well, I'd go to http://www.vcientertainment.com/ and see what they've got.

Then, besides that...

Sony has released Columbia's BATMAN and BATMAN AND ROBIN. Warners has released a 2-fer of SUPERMAN and ATOM MAN VS. SUPERMAN. Image has released the three FLASH GORDON serials. The Roan Group has released THE TIGER WOMAN and a couple of others. Restored Serials has released THE GREEN ARCHER on Blu-ray, the only Blu-ray serial I know of. Lions Gate has released THE ADVENTURES OF CAPTAIN MARVEL.

Alpha has released a bunch of serials, too, of varying levels of quality. Mascot, mainly.

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