Since Monday, traffic has flooded the company's website, causing it to shut down twice.
"We went from being able to keep us up with emails Sunday to now we have 3,000, and we can't even answer them as fast they come in," she said.
Last year, Ground Zero sold 5,500 shelters in 21 states, primarily Oklahoma. The company's most popular model is an underground shelter that is considered safest for those who can manage steps.
(Read more: Six of the Worst Tornadoes in US History)
Following the Joplin disaster in May of 2011, Ground Zero hired additional workers to keep up with demand, expanding to about 80 employees from about 50. Afterward, Schaefer said there were "a lot more companies coming out of the woodwork."
One such company to join the fray is Joplin-based Atlas Safe Rooms, which opened its doors in April 2012.
After the tornado, Jim Moss and several other employees at a local steel manufacturing company saw an opportunity and left to launch Atlas.
"There were some companies that had been doing it for a while, but they were more or less mom-and-pop shops, and they found themselves backlogged by hundreds of units," said Moss, chief executive at Atlas.
Since the steel manufacturing company had capacity to make more additional product, Moss thought a new safe-room company could use the extra capacity to push out product faster than its smaller competitors.
"Our thought was, if you want a storm shelter, you want it now," Moss said.
Atlas currently manufactures only above-ground safe rooms, which enable people to secure themselves quickly when a storm arises.
During its first year, Atlas sold 142 units, but since Monday's tornado demand has skyrocketed, with 40 to 50 new orders coming this week.
Twister Safe, which opened near Joplin in 2004, also sells safe rooms in the area and has also seen an uptick in interest.
"I think our website has been hit about 19,000 times since Monday," said Jennifer McKeough, an independent distributor and consultant. "Last week we probably had about 2,000 for the whole week."
Prior to the Joplin tornado, Twister Safe was the only manufacturer of above-ground shelters for 100 miles. Since then, several companies, like Atlas, have entered the market.
The advantages of the above-ground models include not having to brave the elements to get inside and avoiding groundwater problems that can occur in this some parts of the country.
Safe rooms and storm shelters are especially important in Tornado Alley because many homes lack basements, NPR reported. Fewer than one percent of newly-constructed homes in Moore and surrounding towns extend underground, largely due to the high water table and the red clay that most of the area's homes stand on, which absorbs moisture easily.
In the heat, the clay tends to dry out, causing a cycle of contraction and expansion that adds pressure to concrete-reinforced basement walls, causing leaks.
But NPR noted that even after improvements in building technology mostly solved this problem, people avoid basements largely because of a "psychological hangover for people that are used to seeing houses from the '40s and '50s, when the technology wasn't quite as good for waterproofing."
Buying storm shelters are another thing that Oklahomans typically avoid, but Ground Zero Shelters' Shaefer said many are now taking the plunge.
"People think, "Oh well, we don't have to worry about it. We're not going to get hit," she said. "But now, they say we should have done it, and they're doing it."
—By CNBC.com's Katie Little. Follow her on Twitter