A bank! All the banks offer merchant accounts and will have gateways they are setup with. Authorize.net is the big gateway if you want to do heavier coding and integration with existing solutions but there are hundreds of thousands of options. Regional banks usually offer the best rates. But this is if you are a "real" business. E-proccessors like Stripe, Square, PayPal etc are going after the very low hanging fruit of what are essentially consumers more so than businesses.