My DW, being the accountant she is, makes me shop 'til I drop on the internet for anything. ๐ I bought 4600 series Bilsteins about 5 years ago for our F250 and the lowest price I could find at the time was on ebay. I asked the seller if they could better the listing price and they said to just go to their online store. Someone has to pay for ebay's fees in the end... Sometimes you can tell if a seller on ebay has an online store by their username. Try using the correct part number for the application in a google search.
Make sure you will get the factory warranty when buying from some sources. I believe you need to keep the original boxes to return them in if ever sending them to the factory under the warranty terms & conditions. Chances are pretty slim you'd ever need to do that tho.