It Isn't Based in History
You won't find the answer to this element of your question in that form:
... rather an outside source that shows the historical connection) to
show the origin of scimitar as a druid weapon in Dungeons and Dragons,
and if, as I suspect, the scimitar was a substitute for a curved sword
thought to be used by a historical druid.
That isn't how the scimitar arrived as a standard Druid weapon.
For Dungeons and Dragons, the original source of the druid and his weapons is in Supplement 3 to Original D&D, Eldritch Wizardry. On page 2, it limits Druids to the following weapons:
Daggers, sickles, or crescent shaped swords, spears, slings, and oil.
Most of us playing druids1 turned to the only curved sword we were familiar with, a scimitar. That bit of expediency was formalized in AD&D 1st edition PHB (p. 19, Table II, Permitted Weapons) and became an established trope/feature.
The relationship between the historical sickle2 used to harvest misteltoe and a curved sword was a stylistic point. From the man himself:(note:Gary was posting as Col_Pladoh)
Quote Originally Posted by Heathansson
Hello, Colonel! Hope you're feeling better!!! Just a question that stretches back down the eons to
1e.: why do druids use scimitars? It just seems curious with the
Heh, It is because the scimitar is as close a sword weapon I could come up with to match the druids' mistletoe-harvesting sickle. Cheers,
Druids as a character class were originally a sub-class of Cleric. At that point in the game's evolution, Clerics generally could not use edged weapons. The scimitar was a stark example of how different a Druid was from the garden variety Cleric. The "curved sword" theme related to the sickle was carried over in Unearthed Arcana (1985) when the khopesh sword for druids was added. (Historical evidence for relationships between Druids and Ancient Egypt: a topic for another question?)
Use of the scimitar / curved sword by Druids is not supported in history, per your point in the question (this squares with the modest reading/research I have done on the topic). Many of the OD&D rules, standards, and concepts came from short stories, books, movies, mythology and lore ... often at the expense of historical accuracy.
The Druid began as an encounter/monster in Greyhawk. He led uncivilized / barbarian / berserker soldiers. This comes off as a variation on how the Romans viewed the Celtic warrior bands, put into game form. It is worth remembering how Raw and Unpolished the OD&D material is, and to a certain extent AD&D 1e. A point made by Gary Gygax regarding Druids as a game feature ...
The primary appeal of the Druid class from a creative standpoint is that the Romans were so thorough in destroying them and their religion that we know virtually nothing about either
1 Personal note: I first played a druid in D&D in 1976 who was more inspired by Getafix, the Druid from Asterix and Obelix cartoons, than by any Celtic spiritual leader/wise man in Roman and pre-Roman Britain. I learned about that later.
2 Pliny recorded the use of using golden sickles to harvest mistletoe. Book 16: The Natural History of the Forest Trees. The Druids--for that is the name they give to their magicians -- held nothing more sacred than the mistletoe and the tree that bears it, supposing always that tree to be the robur. The mistletoe ... when found, is gathered ... fifth day of the moon ... Clad in a white robe ... cuts the mistletoe with a golden sickle.