Source for pan gasket - 904LA, also recommended fluid?

I use the reusable Mopar gasket, #4295875AC. No leaks, ever. And like I said, it's reusable.
Cheapest place I've found is
https://www.amazon.com/Mopar-4295875AC-Transmission-Pan-Gasket/dp/B0049EDJ6U?tag=fabo03-20
As far as fluid goes, people are all over the place on that. Go with whatever fluid is recommended by the manufacturer of the bands/clutches.
If it's got the original frictions inside of it yet, I look at it this way- The original factory spec was for Dexron II. The modern, backwardly compatible equivalent of that is Dexron IV; but people run Type F and ATF+4 also with no issues, so pick your poison.