Where have you looked up till now?
The repair parts are seldom seen. You might do better buying an entire Static Excitor. The transformers rarely went bad. So there were not a lot purchased by the army. You might consider having it rewound. Or place an add in SS, and maybe Suprman or someone can help you.
Also keep in mind, that you should not hamper your search by only using MEP-005A. The MEP-004A uses the same static exciter. So when searching, use both model numbers. I have seen static exciters in the big auction site, of late. There are places in the internet that have bought up all kinds of army parts, and sell them. I personally would bump Suprman first.
The positive thing about buying an entire static Excitor is that you get a volt regulater with it. And that part is something that goes bad much more often then the transformer.