Does anyone know if I can book one on line or only through a travel agent?
You can buy through Expedia or Orbitz or any of those sellers. They tend to call them "multiple destination" tickets, but it's the same thing. :)
You don't have to go through a travel agent to book open jaw flights, most of the websites offer that option.
We almost always fly open jaw and have always ordered our tickets on line. We usually but not always fly Air France.
Anita, we always fly open jaw and have done it ourselves. Orbitz and Sidestep have the option, and are the first two sites I go to in order to compare price and availability. (They're basically great search engines to scan all options.) On our trip to France this summer, I went ahead and used the American Airlines website (it was the lowest on the other sites, and I like to book directly with the airline when possible in case of a glitch). In the past I wasn't able to use their site on open jaws, possibly it didn't work with their partner airlines if you needed a connection. Anyway, both types of sites have worked very well for us. Good luck!
I normally use the net to determine exactly which open jaw flights I want, and then book them at a travel agent. There are two main reasons I use that method - first, in B.C. any tickets purchased through a travel agent are covered by the travel insurance fund (I still remember the Canada 3000 fiasco); also the travel agent occasionally finds better or less expensive flights that I wasn't aware of.
Happy travels!