You will generally be able to obtain a merchant account from one of two places:
Authorized Agents
The agent with be authorized and sponsored by a bank (known as a member bank) and will also, in addition, need to be registered with the credit card companies with whom they want to be able to process credit transactions for customers as either a Member Service Provider (MSP) or a Independent Selling Organization (ISO).
Of course there are very strict rules governing the operation of the agent and they will be fully checked out by the member bank before being authorized or sponsored. There are also various fees they must and regulations concerning how they are able to promote the merchant accounts – as you can imagine, anything involving the provision of credit must be dealt with very carefully.
One way to help check if an agent is or may be authorized by the credit card companies with whom they purport to deal is to check for a disclosure statement that will be required on all marketing material produced by them. the disclosure will say something along the lines of: “<Company Name. is a registered MSP/ISO (which is applicable, or both) of FDIC insured <Bank Name>, <address>.
A properly authorized agent may be liable for a fine and other disciplinary action if the disclosure is not displayed and in all cases where it is not, you should tread very carefully.
Processor or Sponsoring Bank
In this instance you will obtain the merchant account directly (as opposed to through a 3rd party agent as above).
The bank will have a credit processing relationship with the credit card vendor and by virtue of this can open merchant accounts directly with vendors/merchants. Such a bank is also sometimes known as a member bank.
Of course the actual criteria and process through which you will need to go through in order to obtain a merchant account with such a bank will vary. Sometimes they are limited to customers with long-standing relationships with the bank already in place, those in the immediate area, or those that have been in business for a certain period of time, i.e. not every member bank that you walk in to is going to let you apply for a merchant account.