Your white shirt needn't be "top notch." It should fit you, be clean, be relatively wrinkle-free, etc. But it truly won't matter that you're wearing a $35 Lands' End white dress shirt, instead of a $250 white dress shirt.
Brooks Brothers was likely used as shorthand for "a traditional, business-appropriate suit." Brooks Bros. is not the only source of such suits. A whole lot of interview suits are purchased at Jos. A. Bank (usually for around 70%, during one of JAB's very frequent sales), and while most JAB suits fail to find much love here on styleforum, in part because they tend not to be as slim-fitting as some people here prefer, I really don't see you losing any points at your interview for wearing a more traditional-fitting navy or grey suit, white shirt, and appropriate tie, shoes, etc.
There are other sources of business-appropriate suits, of course. I simply used Jos. A. Bank as an example, because it's a popular source, and cheaper than Brooks. A little review of past posts should provide you with a number of other choices.
If the book you're reading explains WHY it recommends certain items, pay attention to understanding those reasons. If the book doesn't explain, for example, why you should look for certain qualities in a shirt, why a particular color of suit is recommended, why some necktie patterns are preferable to others, etc., then find another book which does explain the reasoning behind the author's choices. Dressing appropriately is more about understanding the underlying rationale, and not so much about slavishly obeying an author's directions without either deviation or comprehension.
+1 to this. Doesn't need to be top notch--but don't go full on polyester garbage, either. Lands End shirts are decent, as are Charles Tyrwhitt or TM Lewin if the shipping will get them to you in time.