easy selectbox ordering

HELLO. I am bulding onepage website based on WP. i want to make at the end of tis site easy order system. I want to make 2 selectboxes. From first you choose brand and from second you choose the model from selected brand. the third array will be integer for quantity of products. after you confirm order you will be redireced to ad you billing and shipping adress. And there is my question. if there is some plugin which allows something like this . that you only choose brand and after that model of brand. i need to make it easy as possible. Thanx for any answer ??