The link Aaron posted is US-based, as I read the website.
There are actually a fair number of manufacturers of these items in the US. If you do a search on “membrane keyswitch manufacturer,” that should pull up quite a few. What will take you some time is finding the right vendor fit for your business. What I mean is finding a vendor that handles businesses in your size range with similar regulatory requirements (or lack thereof). If you make a low-volume industrial product, you’ll have a very difficult time working with a vendor whose bread and butter is medical. i.e. The medical vendor will drive you crazy with documentation requirements.