There are some airfare prices that sound too good to be true. But you can find them, if you know where, and when to look.

“There are so many cheap flights available even at a time like this when airfares are at relative highs,” said Scott Keyes, founder of the airfare deal site Going.com.

He got hooked on finding low airfares a decade ago after nabbing an unbelievable fare to Italy for just $130 bucks.

“I felt like gosh, even if this trip is terrible, I paid almost nothing to be here, no big deal,” explained Keyes.

If you want to nab a fare like that, you have to act quick.

Sites like Going constantly monitor airfares to find mistakes and unadvertised cheap fares.

“You’re not necessarily going to know about them because the airline isn’t running a big ad campaign,” said Keyes.

You can sign up to get free low-price alerts from a variety of websites, but unless you pay, the information might be delayed until after subscribers get first dibs.

Going’s free plan sends international alerts only. For $50 a year you can get more frequent alerts and domestic destinations.

Dollar Flight Club sends basic alerts for free, but for $69 a year, you get even more deals and discounts.

FareDrop sends domestic flight deal alerts for free and allows subscribers to set more specific regions and fare prices they want to see.

Of course, there’s no guarantee a fare will be there when you go to book.

“The broader you take it, the more flexibility you give yourself, the better your odds of getting a great deal,” concluded Keyes.

I have signed up for free alerts from several of these deal sites and many times I’ll pop the dates and airports into Google Flights. I’m often surprised to see that more often than not, the deals are still available.

Bottom line: if you’re looking for deals and you’re flexible, it’s worth signing up for at least the free alerts, and if you really want to go somewhere cheap, it’s best to pay for premium alerts.