How create a custom Roller Coaster? Where I can get the track pieces?
You can buy tracks directly from LEGO. You go to LEGO service web page and follow steps below:


Click on BRICKS &amp; PIECES just below the banner
Then BUY BRICKS
Provide your age and location (please be aware the list of countries where LEGO is shipping to is limited)
On the left side you have an option to chose parts from particular set. In this case use "10261". 
You might need to click "Show more" a few times to get to individual pieces you need.


Note. 10261 Creator Roller Coaster set does not have every type of element within track system. You may use part numbers provided below to look for individual tracks using the option on the right side in Step 4.

Alternatively you can purchase all the pieces on Bricklink. Look for:


25061 - Train, Track Roller Coaster Curve, 90 degrees 
26559 - Train, Track Roller Coaster Ramp Large Lower Part, 6 Bricks Elevation 
26560 - Train, Track Roller Coaster Ramp Large Upper
Part, 6 Bricks Elevation 
34738 - Train, Track Roller Coaster
Ramp Small, 3 Bricks Elevation 
26561 - Train, Track Roller
Coaster Ramp Steep, 6 Bricks Elevation 
25059 - Train, Track
Roller Coaster Straight 16L 
26022 - Train, Track Roller Coaster
Straight 8L