I know a few small businesses that have delt with supermarkets, and the resounding feed back was be very careful as they are ruthless. They can gear your business up to sell millions of units and then pull the plug in a very short period of time, leaving you totally in the lurch.
One way to get your products into supermarkets is to use middlemen. There are many companies out there that act as food brokers who will supply your product via their network. You will loose a bit of margin but gain security.

If you do a search via google you will get a list of companies.