Depends on what upgrades you’re considering. I’ve had very good experiences with Competitive Cyclist and Jenson. As the previous poster indicated you also can get some good deals on Ebay (be sure to check the sellers reputation) and sometimes even Amazon. But I would give CC and Jenson an opportunity to price match as they are highly reputable and have been great to deal with even on returns.
I order mostly from Jenson because I live in SoCal and whatever I order is on my doorstep the next day with free shipping. That being said, if I’m not in a hurry, I shop around to find the best deals. If you know what you’re looking for, have the patience to deal with the German websites and have time to wait, you can get some screaming deals by buying from the UK or Germany. I get tires for $30-40 that cost $60-80 here. I found pedals at Merlin for $80 that were $130 everywhere in the US. My buddy gets all his SRAM drivetrain parts from Germany as well.