The only site I’m very familiar with is Expedia, but I’d like to do a little more checking if possible. What are your favorite places to book through?
Just in case it makes any difference, I am trying to book a roundtrip from Oklahoma City to Bucharest, Romania in December.
I have also used Travelocity, with generally good results with the exception of one bad experience*. There’s also Orbitz. Also, you should try web sites for individual airlines; some airlines that go to Romania might not be on the big travel sites. You can also try a brick & mortar travel agency. A human being can do things even a good web site won’t be able to do.
*They booked a flight via British Air to Egypt via London and got the name wrong on one of the tickets. For international travel that is a critical mistake. It wasn’t clear whether it was Travelocity or BA who made the error. They sent me a letter telling me to FedEx the ticket back to them at their expense, but they didn’t give me the information needed to charge it to them, and refused to do so in a follow-up phone call. I ended paying about $25.
I use kayak.com.
I’ve had good luck with kayak (www.kayak.com). I’ve never shopped for tickets from the US to Romania, though.
After you search through Kayak, you might also want to check with a travel agent. And be prepared to shop around–different travel agents have different areas of specialization and can sometimes give you different rates for what’s essentially the same trip.
Can you find a travel guide to Romania? If so, the guide will probably have recommendations for websites that specialize in tickets for flights between the US and Romania (or, at least, between the US and various places in Eastern Europe.) Those websites might be worth checking out.
I would try as many as you can and compare.
I’ve used orbitz.com, lastminute.com, travelocity.com and expedia.com
For transatlantic fares I’ve found there can be a huge variation in price. For example, my last visit to Cleveland I could have flown from Dublin via any of about five cities and the price varied depending on which connecting airport I chose.
I have never used a travel agent before - how would I find such a beast? 
I like to use Cheaptickets.com and Orbitz to research fares because they offer flexible search options. If you’re not restricted on the days you fly out and return, but know about how long of a trip you want to take, there is a way you can search a 30-day span of time for ranges of 2-4, 3-5, 4-6, 5-7, 7-10, 10-12, and 12-14 days. You can also choose specific dates and include +/- 1-3 days to expand your search window. Travelocity used to, but no longer allows, flexible dates searches on international flights.
Once I identify the cheapest dates to fly, I check which airline they’re on and usually buy directly through them, as the online travel agencies often tack on some fees.
www.studentuniverse.com (if you’re a student)
www.dohop.com has worked very well for me. We even use it in the office to plan business-trips all over the globe.
I use kayak.com to find the best fare, but then I go to that particular airline’s website to book it.