Ms Cheryl,

Here are two options:
1.Fly from Haikou or Sanya to Chengdu and then take long distance bus at Chengdu Xinnanmen (New South Gate) Bus Station to Leshan. This will cost you much money but save you time.

Cost:
Flight: Sanya to Chengdu USD 166 (lowest airfare)
Haikou to Chengdu USD 218 (lowest airfare)
Bus: around CNY 37.

2.train route: Take K408, K512 or T202 from Sanya to Guangzhou and take K192/K193, K586/K587, 1224/1221 or K827/K830 from Guangzhou to Chengdu. Then take a bus to from Chengdu to Leshan.

Train tickets：
You can get details by using Train Search Here:
https://www.travelchinaguide.com/china-trains/

Bus ticket: around CNY 37.