SELECT 
  DISTINCT warehouse_id 
FROM 
  cscart_store_locations AS sl 
  INNER JOIN cscart_warehouses_products_amount AS wpa ON wpa.warehouse_id = sl.store_location_id 
WHERE 
  wpa.product_id IN (759)

Query time 0.00031

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.00355143,
    "temporary_table": {
      "nested_loop": [
        {
          "table": {
            "table_name": "wpa",
            "access_type": "ref",
            "possible_keys": ["PRIMARY"],
            "key": "PRIMARY",
            "key_length": "3",
            "used_key_parts": ["product_id"],
            "ref": ["const"],
            "loops": 1,
            "rows": 1,
            "cost": 0.00183779,
            "filtered": 100
          }
        },
        {
          "table": {
            "table_name": "sl",
            "access_type": "eq_ref",
            "possible_keys": ["PRIMARY"],
            "key": "PRIMARY",
            "key_length": "3",
            "used_key_parts": ["store_location_id"],
            "ref": ["u428615623_ecartifygonje.wpa.warehouse_id"],
            "loops": 1,
            "rows": 1,
            "cost": 0.00171364,
            "filtered": 100,
            "distinct": true
          }
        }
      ]
    }
  }
}