Some suggestions for you would be:
- 2checkout.com/pricing/
- authorize.net/en-us/sign-up/pricing.html
These are two of the more popular ones for many webhosts. I would choose 2co for their customer service, low cost to implement and no monthly charge. Now if you begin processing large amounts of credit cards it might be cheaper to pay a monthly processing fee with a lower percentage of each transaction and lower per transaction fee.