And Nashbar branded merchandise is pretty good too. Some of their clothing and shoes are very good, their frames, pedals, racks, bags, etc. are very solid too. So is Performance, they're the same company/owned by the same company. If you're not into flashy brand-name stuff then Nashbar stuff is worth looking at. Their SPD sandals are favorite among touring cyclists at 50% of the price of comparable Pearl Izumi sandals, for instance.

And yeah, another vote for Universal Cycles. They're my #1 place to go for components and parts. They're owned by QBP, same company that owns Salsa, Surly, Civia, etc. They make very good wheels too, by hand, takes a couple of weeks but it's worth it. One of QBP brands, Dimension makes value, but decent quality parts and accessories, stems, seatposts, bars, etc.

The main reason that I often buy from Amazon is Prime and fast, free shipping. Amazon also has constantly some of the lowest prices. One big problem with Amazon is their website: for technical stuff, the product descriptions are often incomplete and misleading. If you really want a 12-36 9-speed cassette you may want to go to Nashbar or Universal and pay a few bucks more but get what you need. But something like a Topeak bag is usually cheapest and easiest to get from Amazon.