https://www.siampoolth.com
one of the services that Siam Pool offers is to train you or your staff to care for it. they trained my gardener who had no pool experience and he now does a much much better job than previous teams.
your best bet would be Union on Suthep near the university, but on the other side of street, close to Nimman or World Chemical. i have some here and assume i bought it at one of those two places.