Generally speaking, finding banks and ATMs isn't difficult, especially in towns. Most main bank branches close at 4 pm, with many smaller branches, often located in shopping malls and department stores, remaining open into the evening and on weekends. Remember that the farther out from civilization you travel, the fewer ATMs you'll find. Don't expect to withdraw money outside a national park or in a tribal village. It's probably best if before leaving Bangkok you exchange the money you think you'll require.