Anybody can tell you a bunch of bull to your face. With a online supplier there are ways to see if they are reputable. You normally can get a brief history of the company and the founder. Read testimonials from customers who have had actual transactions with the store. Speak with customer service if you have questions not answered on the site.
Request quotes from several sources, including your dealer. While replacement parts from your dealership will usually be a bit more expensive, it helps to receive a quote from as many sources as possible to avoid getting duped by the 'low ball bid.' Some repair shops under bid to get you in the door, only to add on unneeded services later.