Plugin for SAP Business One and ecommerce website

I'm not aware of a plug-in, so you are probably looking at a custom solution.  You don't necessarily need to hire an SAP expert if this is a one-way data display (i.e.  you are not sending data to SAP, only showing data).  The Business One database is a not a very complicated structure, especially if you are just wanting to display stock of an item (which would involve two tables:  oitm and oitw).  

NOTE:  If you are looking for two-way interaction with Business One, where it's not just showing data but updating data, then you must get an SAP expert.

But I'm assuming that's not the case.  It sounds like you're looking for data display, not updating SAP data.
If you have an existing web site, I would look for someone who is familiar with that technology, so that your new functionality fits with your existing website.  Make sure the person knows SQL, and how to properly implement a search on your data without risking SQL Injection attacks.

Also consider whether or not viewing this information should require a login (do you want your competitors to see this info ?).  If it needs to be secure, make sure the person you hire knows about how to secure your information.

So, in sum, I'd recommend you look for a web developer expert who's familiar with whatever language your current website uses, and familiar with SQL and web security.  Anyone who is good at SQL will be able to easily query your Business One database.

HI zephyr_hex

NOTE:  If you are looking for two-way interaction with Business One, where it's not just showing data but updating data, then you must get an SAP expert.

Yes, we are looking for two-way interaction, we want to create in SAP a new  invoice every time the customer do the payment and we want to allow the creation of new customers too.