Where can I buy large plants in Bangkok?

I assume you have vehicle since you state you want large plants. If you go on Outer ring road, from Rama 2 up to Putthamonthon Road you will see many shops selling large outdoor plants on both sides of the road. You must go on the frontage road for these shops. Also on Putthamonthon Road again on the frontage road both sides going to Nakhon Pathom you will see many of these shops.