Letters and postcards to the United States and Canada cost €0.95 (about $1.30) for 20 grams (about 0.7 ounces). Letters and postcards within France and the rest of Europe (including the United Kingdom) cost €0.63 (about 86 cents) for up to 20 grams. Stamps can be bought in post offices (bureaux de poste) and cafés displaying a red TABAC sign outside.
If you're uncertain where you'll be staying, have mail sent to the local post office, addressed as "poste restante," (there's an €0.85 service charge). Bring your passport along to collect your mail.