I hesitate to give advice about buying coins. I have been at it 25 years and have learned a few lessons, so I feel qualified to give good advice, but what is good advice depends upon your circumstances. You can build a great collection this way. Genuine coins at the best possible prices come from specialist ancient coin dealers . There are numerous dealers in the US who specialize in ancient coins.
Ancients are almost always cheapest from specialist dealers .
I talk about buying on the web below, but I recommend you do not skip down to it. Read the next part about specialist dealers first. I used to visit dealers in U.S. coins hoping to find an occasional ancient coin at a good price, but I spent far more in gas driving to see junk than I ever saved with a good deal. It plays a role, but is often misrepresented. Condition is the tricky one.