Bad Elf makes a small GPS receiver with a lightning connector that plugs into the iPad. Works fine, but slow to initially acquire position. No ADS-B, just position. They also make various bluetooth GPS receivers that are probably better, but if you're in an environment where bluetooth/wi-fi is a no-go, this is the only solution.
iPads with LTE/cell capability have an internal GPS antenna that is as good or better than the little Bad Elf...so if your unit is buying new iPads, spring for the cell capable ones, it's well worth it.
The best way to go is something that gives you GPS and ADS-B in. Stratus is great, we use them...6-9 hr battery life. There's also the Sentry puck, which is ForeFlight's latest proprietary thing. I think the battery lasts longer, some other neat things like a CO2 sensor for GA flying. I don't have much experience with the Dual options, but they offer similar capability, but Bluetooth based. All of them also have a poor man's AHRS system as well.
There's no perfect setup for everyone, depends on your aircraft/mission/restrictions/etc.