The vocabulary that you need for IELTS will vary depending on the subject matter. Here are two websites that give you some good information.

This one has 20 adjectives to start with. They are quite good, but there is no practice so you need to practice on your own in order to remember them properly.

This one has a variety of topics. It gives the word and definition, and there is a link to listen to her read the information with a clear British accent. There is practice using the words at the bottom.