Jetpacks use USB chargers. You do not need to buy one specific for the Jetpack for it to charge. Using a charger with a lower output than the Jetpack can handle will result in slower charging than normal. Using a charger with a higher output than the Jetpack can handle should be stepped down to the max value that is supported on the Jetpack (but I don't recommend that from a safety point of view).
Research the specs on the Jetpack
1. Review the USB jack on the Jetpack
2. Review the Power requirements on the Jetpack (on the battery, back cover or user manual)
3. Review USB wall chargers that comply with the first two steps
Research the specs on the current USB charger
1. Review the Model Number
2. Review the power Output values
3. Search online for chargers that match the model number or the power output ratings of the current charger.
For example my MiFi 7730L uses a 5 Volt/2 Amp USB wall charger. It can also provide a few other output levels too. My phone charger provides the same 5V/1.6A output but also two others for "Turbo" charge capable devices. However the USB jacks on my Jetpack and my phone are not the same so I need to use a different USB cord for both of them, although either charger would work with either device.