my above ground vinyl pool is leaking. Where can I buy Nitrile rubber sealant in Bangkok.

Also there are other underwater sealants and epoxies suitable for this job. But where to buy.

Home Pro or the like generally have a good range of epoxies which bond to most materials some of which will set underwater. I thought some of those vinyl pools came with a 10 year warranty & if so you should be talking to the supplier.