The following is the biggest supplier in Thailand. They have an agent in both Phetchaburi and Prachuap Khiri Khan, but I do not have a contact for them. Both the owner and his son at VVP speak good English, or you could email them.
http://www.vvp4x4.com/
There are 2 shops in Barn Pong that manufacture all types of after market equipment, but neither of them speak English.
https://psk4x4banpong.bentoweb.com/th
http://www.cherdchai-offroad.com/
The old gentleman at Cherdchai is getting past it and really should retire.
The woman that runs PSK is a complete idiot, but they do have good people in the shop and build good products.
I believe they export parts, so should be able to send items to you. Most definitely need a Thai speaker though!
I have never found any shop in Hua Hin.