Where can I buy ancient Roman coins?

Could you please elaborate on the options available for purchasing ancient Roman coins? Are there any reputable online marketplaces or auction houses that specialize in this area? Also, would it be possible to find these coins at local antique shops or museums, or would I need to seek out more specialized dealers? I'm particularly interested in knowing about the authenticity and authenticity guarantees that come with these purchases, as I want to ensure I'm getting a genuine piece of history.