Often times I hear people say "I got it offline" to mean they downloaded it from the internet, is this an accepted term or should it be "I got it online"?
In isolation, I got it offline is probably incorrect, but I can imagine contexts where it would be correct.
In standard usage, online means on or connected to the internet (or, more rarely, intranet etc.), and offline means not connected to the internet, or just on a local computer (whether or not it’s connected).
So since when you download a file you must be connected to the net, it would be more usual/correct to say you got it online.
However, think about how they got the groceries home can mean they (purchased the groceries elsewhere and then) brought them home. Analogously, I could imagine someone saying Normally I work with my spreadsheet online, but I got it offline this morning so I could work on the plane. In this context, I got it offline is being used clearly and not unreasonably to mean downloading something — getting it from online to offline. But this example is a little contrived; in most situations, it should be I got it online.
I'd say that to convey the meaning correctly, you either got a file from an online resource or else you downloaded a file for offline access. "Got a file offline" as an isolated expression to me sounds like you deleted an online resource. For example: I put a few racy pictures of myself online, but now that I think things through, it'd be best if I got them offline.
Casually, perhaps go with that you got it off the internet.