Hello, I work for a billing company that expanding into Payment Processing business. What I need is payment processing software, server and client sides (2checkout.com for example), but can't find any for sale. Feels like no one selling or developing this kind of software, at least I couldn't find anything googling for the two last days.

If anyone knows where payment processing software sold or developed, please share some links or names. I need a proven to work solution, because one little security hole can cost you heavily in this business.