How can I find a service provider that can help me develop and host an e-commerce website while fully integrating with a POS system at my store?
Well, it certainly looks like you've done your homework and you know what you need. You've gone the route of finding great web design companies, but they were short on the POS end.
I would try attacking this problem from the other end by reaching out to POS companies and getting them to refer you to two to three web-design companies that work well with them and their products.
Sometimes you have to turn a problem completely around to solve it.
Hope this helps,