Just search for ublox NEO-6M or NEO-7M or NEO-M8. Ebay items have always worked for me, just avoid the super-cheap sellers. They're the ones that might be counterfeit or might not perform to the specs.
Amazon is probably similar... avoid the super-cheap "marketplace" sellers.
I've heard one bad report about banggood shipping a different brand from what was ordered.
Adafruit modules are popular, too. They use the MTK chipset, but their Arduino library is the worst. I've compared their library (and others) with my library, NeoGPS (see docs). NeoGPS is smaller, faster, more reliable & accurate, etc. It's available from the Arduino IDE Library Manager, under the menu Sketch -> Include Library -> Manage Libraries.