No need for a real TA. Just hop on google.com/flights and play around with dates. UA/NH and AA/JL offer these pretty frequently and you can book the whole thing from Google flights. This type of thing is fairly routine for those two airlines so Google Flights is more than capable of handling it. Here is one such example with random dates and using NYC as your starting point and pricing out at ~$1150 with the stopover:



From there you can easily click through to the AA/JL website or UA/NH website (if you choose UA/NH) or to an OTA (in this case it's linking into Expedia). Just pay attention to which Tokyo airport you are flying in and out of on your layover (NRT vs. HND) -- shouldn't matter as much on your flight home from BKK (just make sure you are connecting via the same airport and don't have to switch).

Pricing between AA/JL and UA/NH is virtually the same on the major city pairs. Unless status is relevant, I would personally advise you go with JL if flying Y which is still 9 abreast on the 777 and 8 abreast on the 787 vs. NH which has moved to 10 abreast on their 77W and 9 abreast on the 787. Likewise, AA and UA are 10 abreast in Y and well it's AA and UA.