It isn't known where the rest of the Avengers are during the kidnapping. Since the entire event happens in the space of about eight hours, it could simply be considered too difficult to gather the Avengers in that short space of time.
Thor lives in Asgard most of the time. Until they repair the technology/magic which creates the Rainbow bridge, I suspect commuting is out of the question.
The Hulk: As seen in the Avenger's movie, it is likely the Hulk spends his time as far away from other people who know who he is as possible. An eight hour window probably makes getting his attention challenging. Nor is he known for his subtlety. The last thing you want is HULK SMASHING his way toward the problem you are trying to sneak up on.
Hawkeye and Black Widow are both SHIELD operatives and could easily be on assignments anywhere on the planet. While they might be perfect for the job, they could just as easily be in Kazakhstan doing a secret operation.
Captain America, currently a fellow out of time, is probably being brought up to speed and likely living in a SHIELD base while he catches up on the last sixty years of history. He would likely be very concerned if he knew of the kidnapping but no information had been made public at the time, so he would not be mobilized either.
Most importantly, in the comic genre, it is common for a superhero to be alone on a mission even though the world is slathered in potential metahumans who COULD be available but aren't. We are supposed to accept the idea superheroes are always engaged in something which occupies their time and makes them unavailable to the star of the comic we're reading. Comics poke fun at this sometimes when they have the Avengers call the Fantastic Four and Reed Richards answers from the Negative Zone saying he wishes he could help but they have their own problems.
Kevin Feige, Marvel Studios President stated in an interview:
Journalist: After cell phones came out horror directors had to come up with elaborate ways to explain why people wouldn’t call someone on a cell phone for help, now that all The Avengers know each other do you have to come up with excuses for why Tony Stark wouldn’t reach out when he needs a hand?
Feige: It’s a good question, and it’s sort of half and half. I am betting that like the comics you don’t have to keep – if you are reading a standalone “Iron Man” comic, they don’t spend every page explaining where every other Marvel hero is. The audience kind of accepts that there are times when they’re on their own and there are times when they are together. I’m betting that movie audiences will feel the same way. That being said, there is a little bit of lip service here and there to that. There is also just the very nature of Tony wants to, once he barely survives that house attack you saw today, and even you saw it in the message he left for Pepper, he’s basically saying “I’m going off the grid to try to figure something out.”