Which selection/proficiency test should I use?

I am conducting a research on some ESL students on their reading comprehension and vocabulary acquisition. They will be divided into three English proficiency levels- low, intermediate and high. I need a selection test to place the subjects into the three proficiency levels. I've looked at several tests, eg. Nation's vocabulary levels test(but this is only on vocabulary), Nelson Denny's Test (this is for college and university levels) and IDEA proficiency Test. The last test seems to fit but it has to be purchased, but can't be done online. Would appreciate very much if anyone can offer advices on other suitable tests. Thanks