When to plant broccoli and cauliflower in North Thailand as both are cool weather plants but like sunlight.  When is the best time plant seeds?

I'm not a hundred percent sure, but I'd say now, just as the rainy season is coming to an end (in about a month or so, at least for us here in Isan) and then the temperatures start to drop during the day and a lot after sun set. 

You can try. I did, didn't work, not surprising that you can't find seeds easily. 

With time, I just gave up on insisting on growing the stuff I grew back home and found substitutes, maybe not the same taste but vastly superior in terms of vitamins and mineral. 

Planted a few weeks ago now ready to replant outside !!