Here is a list of all Pokemon who are or whose evolutions are capable of Mega Evolving that you can catch in Alola, in Sun and Moon, before the post-game.
Slowpoke at Routes 1, 15, 16, and at Kala'e bay.
Abra at Route 2 and Hau'oli City.
Gastly at Hau'oli Cemetary, Memorial Hill, and in Poke-Pelago.
Magikarp at Routes 7, 8, 9, 13, 14, 15, Melemele Sea, Kala'e Bay, Seaward Cave, Paniola Town, Brooklet Hill, Secluded Shore, Tapu Village, Malie Garden, Poni Wilds, Poni Breaker Coast, Seafolk Village, and Vast Poni Canyon.
Sableye at Ten Karat Hill and Vast Poni Canyon.
Bagon at Route 3 and Kala'e Bay.
Kangaskhan at Wela Volcano Park.
Pinsir at Lush Jungle and in Poke-Pelago.
Beldum at Mount Hokulani.
Gabite at Haina Desert.
Absol at Route 14, Tapu Village, and Mount Lanakila.
Snorunt at Route 14, Tapu Village, and Mount Lanakila.
Sharpedo at Poni Breaker Coast.
Hopefully this helped.
Please let me know if I missed anything that is available before becoming Champion, or if my information is incorrect.