Please view this resource for compatibility of external GPS receivers with the Collector for ArcGIS App:
Post if you have a specific question about a particular receiver not on the list.
Depends on the required accuracy for your project. Eos Positioning offers various high accuracy gnss gnss receivers (from submeter to centimeter rtk) that are all Apple certified and fully compatible cross-platform with Collector (iOS, Android, Windows). Visit www-eos-gnss.com for more info.