Of course technically all starters are available when you choose in the beginning but I'm talking about from events and such.
(In the comments there is a list that's more clear then mine so yeah)
all I know:
all kanto starters (black market in carotos (charmander), somewhere in aquamarine cave with treats (squirtle) and a kakori village help quest (bandits at large) (bulbasaur))
all jotho starters (trade spinda for chikorita on route 2, chrisolia hotel in the pool with treats (totodile) and in carotos mountain if you have magma drift (cyndaquil))
all alola starters (dull key needed (as well as strenght and rock smash if I'm correct) on junction bridge (litten), popplio if you beat that trainer I forgot the name of on route 6 (dive needed) and tylaric falls in terajuma jungle (rowlett))
treecko (in sheridan if you have surf)
mudkip (in sheridan wetlands)
turtwig (finishing all west gearen quests)
piplup (evergreen forest w/ anju's pendant)
oshawatt (in azure shore)
tepig (valor mountain, where you freed reina)
chespin (from Cairo when you have an exact amount of red essence)
fenniken (14+ badges, you need to give a wand to the girl who was talking to braixen/delpha in kakori village. Wand is somewhere in the place you fought ryland/got his badge)
froakie (in akuwa city and as an award in the aquarium quest)
sobble (In route 4 after either 14 or 15 badges)
is this all that is available or are there some more I can get? I'll add it to the list if there's more.
EDIT: Thanks fore everyone that commented!