If you’re not worried about it being exact OEM right now, a company called Masta Performance makes nice locking dipstick replacements. Masta actually makes them for some of the aftermarket companies, and you can save a bit ordering them direct without the various brand names. It doesn’t look like they have a picture on their website, but they might be able to provide a picture first if you want, or it’s possible I might have one here i can take a picture of later. Here’s a link to their website:
https://mastaperformance.com/transmission-dipsticks** edited to add picture **
Here’s what the Masta Performance FE C6 locking dipstick looks like
Also, again not an exact original, but I know that the ‘70’s FE 2wd pickup truck C6 dipstick & tube work in a ‘67-‘68 Mustang, my guess they might fit the ‘69-‘70 also. That might be a little easier and cheaper to find than the original FE Mustang part.