Found via a page of the Worldbank there are two maps depicting the percentage of poor people at subdistrict level. The two maps - one showing the urban (municipal) areas while the second the rural (non-municipal) tambon are taken from the paper "Spatially Disaggregated Estimates of Poverty and Inequality in Thailand by Healy, Andrew J., Somchai Hitsuchon and Yos Vajaragupta published in 2003. The paper includes further maps using different ways of identifying poverty - by consumption as in the maps picked by Worldbank, and also the income inequality by the Gini coefficient.
As the maps down to subdistrict level are hardly recognizable except for those really large subdistricts in the mountainous areas, the maps showing an average of the rural data to the province level are much better to recognize. I have redone those maps (Figures 16 and 17) using my boilerplate map to display them here - the upper one the ratio of poor people, the bottom one the income inequality. Not much surprisingly, the poorest areas (red and orange) are the northeast (Isan), and the three deep-south Muslim province, and only in central, eastern and some southern provinces the percentage of poor people is below 5%. For the Gini coefficient the picture is less clear, even within the Isan here are areas with a value below 35 and thus a income inequality comparable with western countries. As of the 2007/08 Human Development Report Thailand in medium has a Gini coefficient of 42.