Confusion on replacement fob

One of the fobs went missing months ago and I need a replacement. I am very technical and I don't feel like paying hundreds of dollars for this. I have a 2012 Prius and may have access to TechStream, as I have a relative that owns a mechanic shop (gotta check on that). I'm a little confused on what is needed to program a new fob vs a used fob. Also, I see a lot of aftermarket fobs. While I'm not keen on aftermarket fobs, if it actually works and it is cheaper I will consider it. I am unsure about those because many say they need to be programmed by a locksmith with special hardware, not by a dealer with TechStream. It seems a used fob is doable too. I'm also not sure where to get a new fob that isn't as expensive as getting one from the dealer. It seems easy to find aftermarket ones...