account or a third-party payment processor.
Having your own merchant account will provide the lowest payment acceptance rates - currently no more than 2.19% + $0.25 per transaction. In order to set up your own merchant account in the US, you will need your organization to have a legal presence here. In other words you will need at least a subsidiary to be registered in the US and to have a bank account and address here.
If that is not an option, you can open an offshore merchant account, which will allow you to accept payments in the US, but at substantially higher rates - 5% - 6% or more per transaction. The upside is that you can set it up without having to incorporate in the US. To locate offshore merchant account providers, simply Google "offshore merchant account providers".
The third option is a third-party provider like PayPal and they will not be cheap either. You can visit their pricing page and see if you can find your rate. It might be lower than an offshore merchant account but the downside is that they tend to overprotect and freeze accounts for no good reason.