Even a device no larger than a cell phone can allow another person to remotely monitor a car or truck's location in real time. Hunter said the system can be set to update locations once every one, five or 10 seconds and is accurate up to eight inches and a quarter of a mile per hour.
But he said that although most of their customers are businesses, they often fill orders for individuals eager to catch a cheating spouse.
Once, he said, his company received two orders from the same address on consecutive days. It turned out that both a husband and his wife suspected the other of cheating and unknowingly had two devices installed on the same car.
But other companies offer prying partners even more options.
An aspiring 007's dream, SpyAssociates.com markets everything from a "SpyHawk" real-time GPS tracking system and "StealthCam" alarm clock that hides a video camera, to a nearly $2,000 counter surveillance professional package.
"Greed, lust and fear are the three high-growth industries and this covers all three," said owner Jeffrey Jurist, adding that his clients include law enforcement, individuals, private investigators and corporations. "Everybody's watching everybody. It's just a matter of whether you're aware of it."
While the most popular items are currently GPS tracking systems and covert hidden cameras, he also sells listening devices and computer software that secretly saves keystrokes, Web site histories and e-mail messages.
SpyAssociates also sells counter surveillance technology, such as bugging device detectors, transmitting video camera detectors and audio jammers that counteract listening devices.
But Jurist acknowledged that people who use this technology need to pay attention to the law.
"There's a fine line depending on how [it's being used] and who is using. It's up to the end user," he said, emphasizing that the law differs from state to state.
Jurist and others who sell spy tools say their cheaper, smaller devices let just about anyone with a modest budget accomplish anything they want.
But some of the targets of that technology maintain that it's an unfair, and often fearful, reality that they make possible.
"I feel it's unfortunate that a lot of these companies promote the technology for the use of tracking your spouse or finding a cheater, because what they're ultimately saying to someone is that it's OK to do this," Sherri, 39, a victim of technology-enabled stalking, told ABCNews.com. Sherri asked that her last name be withheld to protect her identity.
After Sherri tried to divorce her abusive husband of 10 years, she said he started appearing in random places at random times.
Three days after quietly changing jobs, he showed up in the parking lot of her new office. A month later, on her way to the airport, she spotted his car behind her.
"Things started happening that were not coincidental," she said. "I knew immediately -- right away -- that he was monitoring my movements, my whereabouts [and] my activities."
After six months, frightened for herself and those around her, Sherri sought out a victim's advocate who helped her see the patterns in her estranged husband's actions.
Ultimately, with the help of a detective, Sherri discovered that her stalker had installed a GPS-enabled cell phone near the radio in her car's dashboard.