Best way is to talk to real specialists (web security experts). DIY in the solution especially if your outsourced programmers lack ethical hacker skills.
Let me know if you are interested.
There are plenty of security consultants/firms that will do code audits.
I can't recommend any, but perhaps if you ask around your tech and web savvy friends. You can search for them too, but then you have to figure out whether they are good at what they do.
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