Not so budget, but here's a great map with a comprehensive guide to Paris shopping. Much more info avail. at the link.
Top 12 Pastry Shops in Paris | WORLD OF WANDERLUSTWORLD OF WANDERLUST
Best Local Paris Stores - Where To Shop In France
A list of the best Pastry shops in Paris: the top 12 Pastry shops in Paris.
The Best Parisian Patisseries Are All on One Delicious Street - Vogue