Felipe, if you drive 1 to 1.5 hours from Bangkok to Rayong you can get much better fruits, lot cheaper price, and real nurseries instead of giant Chatuchak flea market. It's true though that Thai nurseries have very little cultivar selection. For example, with durian it is very difficult to find anything other than Monthong or Chanee to buy.
You can say what you want about the or-tor-kor market but the fruit is the best of Thailand, everything is A-quality and the biggest size. It is expensive but they really have mango/durian/mangosteen all the time and all is very good quality, you won't find it better.
Here is a list of some fruit festivals around may-june.
http://www.sawadee.com/thailand/festivals/festival5.htmlNot all the chomphu are syzygium samarangense, the chomphu phet si rung has big pits but is also very sweet. You could easy buy the plants on chatuchak, there are official shops as well, one sells chomphu tree's from some famous farm/organisation. They also speak english and have pictures of the fruit hanging on their tree's. That shop doesn't come every week though, they visit agriculture fairs all over the country.
We still have sweet chomphu's now but season is almost over i guess, they got me hooked on them and i have 8 tree's now

But those green ones on your pic i only saw/ate once. The tree's are expensive of that one so i still hope to find it cheap somewhere else.
I still haven't seen the giant chomphu's though, they are so new that the fruit is not on the market yet, at least not in Bangkok. Maybe they go to Hongkong or so. Those new ones weigh 250-500 gram a piece is what the vendors claim and are also sweet.
But growing chomphu is not that easy, my tree's have lice and aphids and caterpillars all the time.
There are also nice malay apples to be found but they are rare. I ate a nice variety called "hawaii" on chatuchak and bought the tree.
If you are still in thailand then you have to taste as much as you can. If a fruitvendor is very busy then you should go there. Mango in supermarkets is not as nice as on local markets. Try the namdocmai green if you see them, for me they are the best ones.