Cycles Laurent on Boulevard Voltaire.

http://www.cycleslaurent.com/

Don't get too excited, though. There aren't really many good road bike shops in central Paris.

Regarding riding, I can't help - all of my French riding has been done in the Alps - but I'd imagine you'd have to go a pretty long way outside of Paris to get some decent countryside - perhaps 10-20 miles, and it would be flat.