Naomi , there's no need to deal with Flight Center. Remember, they are essentially a travel agency, and their aim is to earn commissions. They have no interest in saving you money by steering you towards discount airlines.
Use the websites already mentioned to find the flights you need, then book on that airline's website. Many discount airlines fly out of smaller airports rather than the bigger international ones. You'll find flights out of CDG and Orly as well as BVA (Beauvais). But make sure you know what you're buying. Read up in advance about their luggage restrictions (checked luggage is always something you'll pay extra for). EasyJet flies out of both CDG and Orly. Ryanair flies out of BVA.
You already know from your other thread that I don't recommend flying to Paris from London. The Eurostar makes more sense, especially if you book early enough for a discounted ticket. Paris to Venice by air makes much more sense. You'll find lots of options for that one.