My two cents on this one.
The verb "ritrovare", in its reflexive form "ritrovarsi", means, roughly, per se in this context, "to find oneself in some place without having explicitly wanted such thing to happen"; quoting Treccani:
- [...] b. [...] accorgersi di essere capitati in qualche posto, senza averlo esplicitamente voluto o senza aspettarselo: Mi ritrovai per una selva oscura (Dante) [...]
Apologize in advance for the bolded text, which always makes me picture OPs like they are screaming, but I believe that here resides the key for the correct interpretation of "per" in this context, and I think that to remark it it's necessary: mind that "ritrovarsi", whilst it indeed implies the subject having been in movement until the action of "ritrovarsi" itself takes place, doesn't imply, anywhere, the subject being still in movement whatsoever, because "ritrovarsi"'s meaning itself (as mentioned by Treccani) means else, and addresses the fact that the subject realized and became conscious of the fact that they are in some place they didn't explicitly wanted to be in, and hence really says nothing about the subject being or not being in movement; so thinking of the subject as being still in movement at the time of this happening would be rather an assumption.
Now, on the following "per": Treccani mentions 13 different acceptations of "per" (excluding variations); I went through all of them, and, for the above, I believe that this is the acceptation that best describes the meaning of "per" in this context (I italicized the relevant part):
- [...] b. Per indicare il luogo entro il quale avviene un movimento (compl. di moto entro luogo), senza riferimento alla direzione di tale moto: passeggiare, girare per la città; andare, viaggiare, cercare per monti e per valli. Acquista in simili casi funzioni analoghe a quelle della prep. in, e ciò avviene anche con verbi di quiete per indicare una distribuzione entro una certa area: le quali [macchie] nelle braccia e per le cosce ... apparivano a molti (Boccaccio); avere dolori per le ossa, per tutto il corpo.
Roughly translated: "[...] It acquires, in similiar cases, functions comparable to those of the preposition "in", and this happens also with "verbi di quiete1" to indicate a distribution within a certain area [...]".
So, in my opinion, being "ritrovarsi" not a "verbo di movimento1" but rather a "verbo di quiete1", and being Dante not in a specific point of the dark forest but rather vaguely somewhere inside it, this is the acceptation that best describes the meaning of "per" in this context; what comes after in the plot, such as Dante being wandering through the dark forest, should be excluded from the interpretation, as it just comes after.
Hence, for the above, coming to the translation, I'd either second "within", which I think has the privilege over "in" to also express that the subject is inside a certain boundary, or, more simply, just "in".
1: "Verbi di movimento", as opposed to "verbi di quiete", quoting Treccani, "express in various ways a change of position of an entity from a point to another in the space or, figuratively, in the time.