Where can I buy an acoustic guitar D string ?

Assuming your acoustic guitar is a steel string ("acoustic" just means it's not electric), you're looking for a wound single string for acoustic guitar. The gauge of the D should be about 0.032" if the rest of your strings are "light gauge" (which is what most ship with these days). The bronze alloy used to wrap the string should ideally match what's on there, but the new one will sound brighter anyway so no big deal. Phosphor bronze like the D'Addario PB030 ought to work fine. It's identical to the D in their EJ-16 sets, which I've been using for many years.

With shipping charges you might be better off just going to a music store as Mikey suggests - quicker, too. They'll sell you something that will work fine, and people usually don't worry as long as they get something reasonably close, when replacing a broken string. You'll be changing the whole set soon enough.

If your strings are more than a couple of months old you should just replace the whole set now. Pick up a spare set at the same time.

A music store is the obvious place. On-line, why are you going on Amazon and ebay? Why not just Google "guitar strings" and you'll get hits from stores that will sell guitar strings, on-line by mail order.

Every music store in the US that I've ever been to sells individual strings. Have you tried your local music store?