Hello Marcos,
In Bangkok, I would suggest that you make contact with Khun Tri at Bike Zone. He, and the Bike Zone shop are a great place to start your search for all things related to Triathlon here in Thailand.
Here is a great article with contact information for Bike Zone shop in Bangkok - http://bicyclethaila...one-in-bangkok/
If you get down to Phuket and need a great quality road/triathlon bike, go and see Martin or Kurt at Siam Bike Tours in Thalang district of Phuket.
Their address and contact details are here - http://bicyclethaila...vince-thailand/
Also, you can visit their website here - http://siambiketours.com/en/
In Rayong, I would suggest that you make contact with the Rayong Bike Tours Cycling Team. One of their members may be able to point you in the right direction for a road bike or triathlon bike rental in Rayong. Their contact information is listed here - http://bicyclethaila...ubs-in-thailand