Craft beer and ale can be found, but you will certainly pay premium prices for it.
Craft on Soi 23 is great. Only a short walk from Soi Cowboy, with a big open-air area on the corner of the street. They had loads of ales on tap, and you can also take the tasting option and have 4-6 flights of different beer rather than one pint.
There’s a very similar place at the top of Soi 11 called Brew. It’s a lot smaller, but they have a similar beer menu. It’s in the same place where you get the amazing Daniel Thaiger burgers, so well worth a visit!
Beats’ suggestion of the Ekamai Beer House is also a good shout. I stopped there last year and had a couple of great pints. From there I went on a mission to find the Bangkok Mikkeller bar, which is hidden away in a residential area of Soi 63. Well worth a visit if you love unique beer, but be warned... it ain’t cheap!