Single tickets or carnets (packages of 10 tickets) can be purchased at any Metro station and also at tabac (tobacconist) shops, which are licensed by the government to sell them. Single tickets cost about 6 1/2 francs first class, 4 1/2 second (the Metro has two classes); carnets are sold at discounted rates of 42 francs first class, 27 1/2 second class. There are also special two-day, four-day and seven-day first-class passes that allow unlimited travel on consecutive days by bus or Metro during that period.
All tickets can be used interchangeably on buses and also on the Metro (within metropolitan Paris). Route maps at bus stops and inside buses indicate the number of tickets that must be stamped upon boarding according to the distance being traveled.
Although you can pick up the 72 bus at any stop marked with a 72 orange and yellow emblem, getting on at either end is advisable if you want to be certain of getting a window seat. Otherwise you may have to stand, and you will see the base of many monuments but little else. The far back seat is a great place for a larger group to see the sights coming and going.
There are any number of other scenic bus routes in Paris. Try the 52 from the Opera to Pont de St. Cloud, with a pass by the Arc de Triomphe, or the 70 from the Ho tel de Ville through the Left Bank (which should really be seen on foot) to the Radio France building, where you can hook up with the 72.
There are a number of transportation guides to Paris available that will help in planning your own trips. One of the best is the pocket-sized, red-covered "Plan de Paris." It has maps of the city's 96 bus lines, as well as district-by-district maps of the city and a Metro guide.
A few tips: Avoid the bus during the rush hours. Mid-morning and early afternoon are probably the best times to ride. And once you have stamped your tickets, hold on to them in case an inspector comes on board to check.