Eggs are some of the most versatile foods to cook. They are used all over the place. As many have suggested.
But. Yeah it is very unlikely you will find anyone selling one. As almost everyone expects to buy in multiples of 6. There is not the demand for shops to open a box.
Your best bet is to talk to a few niebours. Odds are one of them is happy to sell you an egg if you explain why.
And you never know. You may make a local friend interested in cooking.