Does such a combination of grass seed with weed killer exist in Thailand.

During this past monsoon season large areas of grass has been taken over by weeds of various descriptions.

Not to my knowledge. Regular mowing will get rid of your weeds. The grass loves being mowed. The weeds hate it

Seed and herbicide is not a likely combination. You may be thinking of fertilizer and broadleaf weed killer formulations, but they are not available in Thailand as far as I know. Keep your lawn healthy as suggested by afarang, with weekly mowing at 2- 3 inches height (don't scalp it), daily deep irrigation during dry season and nitrogen fertilization every 6-8 weeks. Manual weeding may be needed, cheap labor makes this a good option, fill in bare spots with new sod. Or start over and dig up existing grass and weeds, prepare good soil with plenty of compost and some aged/dried manure, lay new sod. Ya Malaysia is good at squeezing out weeds, but does best with partial shade. Grass seed may be available, but the weeds may out-grow the grass while getting started; sod is a good way to go if prepared, layed and maintained properly.