Where can you get a husky battery dmu6-3 for a flashlight?

Unfortunately those batteries were discontinued.

They were made for the Huskey hand held spotlight and they have since changed designs and battery styles (Spell that "Planned Obsolescence") your best bet is find a local hobby store that does R/C cars and planes. most of those guys build and/or repair Lithium battery packs for customers, see if they will put together a 6 volt pack using rechargeable AA size with a pigtail that will connect to your set-up. It will be a hair smaller and will require some additional foam padding to keep it secure

This does not have the longevity of the lead acid original, but makes it serviceable, or, if your handy and knowledgeable about balanced charging battery packs you can put one together using an old dewalt or milwaukee "A" style batteries from one of there lithium style packs which fit nicely into the space of the old DMU style.

But, And I can't stress this enough, don't attempt that unless your familiar with that style of pack as lithium batteries are somewhat temperamental to heat and are unforgiving if assembled wrong