Where can you buy cheap wine and spirits in Thailand?

You can't. Go to Cambodia instead. In Siem Reap there is plenty of international wine to choose from. It seems a bit cheaper than in Thailand. More significantly the spirits are dirt cheap. In Thailand I bought a 750ml bottle of Bombay Sapphire Gin for about 945 Baht. In Cambodia, the same bottle was $12.50 (about 400 Baht) and 1L bottle was $US14 (450 Baht). At that price I had to get 2. Similar prices for all other spirits I saw. If you or friends visit Cambodia, its a perfect opportunity to stock up the liquor cabinet.