Problems with hollow replacement buttons iPhone

I work at a phone repair shop, and we are having problems finding proper replacement buttons for iPhones. Especially iPhone 4- and 5-series.

The problem is that the buttons we find, all seem to be hollow or semi-hollow. I mean, where the original buttons are solid, the replacement ones are like empy shells with open holes of varying depths right where they are supposed to touch he phone's internal contact points.

This basically makes them useless, as a phone fitted with these buttons will not be able to change the volume.

Does anyone but us experience problems like this?

Does anyone have any smart solutions or fixes to this problem?

And does anyone know where to get proper "non-hollow" buttons?

Thanks in advance!