You're in luck! The Island Scanning feature allows you to have access to many non-Alolan Pokemon and very early in the game, you have access to all three of the Johto starters.
To perform an Island Scan, firstly, you must first scan and scan ten QR codes, which will register ten Pokemon into your Pokedex. QR codes for every Pokemon in the Alolan Pokedex are available here. Once you have scanned ten forms you haven't seen, you should be able to perform an Island Scan.
Head to Route 2 on Melemele Island on a Friday and perform an Island Scan. Once you've performed it, head to the wild grass on Route 2 and encounter your Chikorita! Note, you can only perform an Island Scan once per day.
Every day and every island has a different Pokemon that can be detected, which are listed here. Furthermore, every Pokemon has a set location that it can be found, which is stated when you perform the Island Scan.
If you want further clarification, watch this lovely video and listen to the person's lovely British accent.
Hope I helped. :)
Source: All the links above