Instructions
Look at Civil War gun replicas. Replicas are made to look like the real guns and are often much less expensive than the real guns, sometimes getting as low as $200 for the rifles or $70 for the revolvers.
Try online auctions. Auctions can end up expensive or cheap, depending on who else wants the item and where the bidding starts. The problem with auctions is that while it can get some good deals, there is always a risk of prices rising higher.
Look at re-enactment websites or groups. These groups often have options available that range in prices. Many of the less expensive guns available from these groups are replicas rather than real guns.