Some background artists, after many years in the industry and knowing their type, make specific wardrobe purchases, like: nurse, cop, or security guard uniforms. Just know, that in many of these castings, they will have that specific wardrobe for you.
Your best bet is to sign with an established extras casting company and ask for feedback from them. You may also want to ask advice from the established background artists you meet on your first few jobs.
For more tips and articles by top entertainment industry career coach, The Greenlight Coach, visit www.TheGreenlightCoachBlog.com