I grabbed a 'Refurbished' USB-C -> Lightning ages ago, was £10. Pretty sure it's just an open box as refurbishing a cable? Anyway, cheap enough not to really complain about...
A look on Apple's website shows this:
"
Charging devices
Thunderbolt 3 ports can also supply power to attached devices.
MacBook Pro (15-inch, 2016) and MacBook (13-inch, 2016, Four Thunderbolt 3 Ports) can power two devices that use up to 15 watts, and two more devices that use up to 7.5 watts. It doesn't matter which side of your MacBook Pro that they're connected to—power is delivered on a first-come, first-served basis.
MacBook Pro (13-inch, 2016, Two Thunderbolt 3 Ports) can power one device that uses up to 15 watts and one device that uses up to 7.5 watts.
"
So it can provide 15w. Which makes sense considering it's connected to a power supply, which is supposed to be being used to power the computer not pass through the charging port. You can however plug your device straight into the power adapter to receive the 'fast charge'.
As for buying another adapter, these work
https://www.amazon.com/AUKEY-Charger-Delivery-Nintendo-Switch/dp/B01LX063QN/?tag=macrumors-20 and really aren't expensive.
It's worth noting that there isn't really any significant gain from using a higher wattage adapter though. 5w is definitely slower, but the difference between 12w -> 29w is marginal considering if you were going to carry around a second adapter. 18w appears to be the 'sweet spot', with 12w being ample if you were already carrying an iPad charger with you. However, the 15w provided through the MBP is right between these and considering you wouldn't need to carry another charger with you, you'd likely find the benefit of just spending the £10 on the cable...
http://gearsofbiz.com/opinion-apple...g-power-adapter-with-the-iphone-update/109002