Most portable chargers will have the USB-A port, so no need for USB-C compatibility, just use your charger cable (USB-C side goes into your phone). Most of these, however, still ship with a smaller micro-USB cable that's more suitable for carrying around; if that's the case, you can always use the micro-USB-to-USB-C adapter that came in the box with your phone.
If, however, you want Fast Charging, you have to look out for that specification on the charger (either Samsung Fast Charging or QC 2.0).
I'm using one from KMashi and it's pretty good. No Fast Charging or included USB-C cable, though, but I use an adapter and that's it.