Reformed literature in Thai is very hard to find.  If we define the term "Reformed" somewhat broadly, then some Reformed titles may be found at a various Thai publishers.  A few of those titles don't seem to be translated well, so I can not recommend them with confidence.

However, I can recommend the following titles from Kanok Bannasan (OMF Publishers Thailand):

The Holiness of God by R.C. Sproul
Concise Theology by J.I. Packer
Pilgrim's Progress by John Bunyan
Today's Gospel by Walter Chantry

You may contact Kanok Bannasan to inquiry about ordering.  It is generally easier to order these titles from within Thailand.  If you need to order internationally, email Kanok to ask what arrangement can be made.

The Westminster Shorter Catechism in Thai is also available for download here.