I've used Majestic Honda online, Bernardi Honda Online and Hardtopguy.com for many purchases and have been pleased with pricing and shipping. Get your oil filters by bulk for much lower prices than at your local shop.
For factory accessories I always go with one of the online dealer shops. I used some of the vendors listed above to get parts for my '07 Accord. Another good one I've bought from is accessories-honda.com
For my '16 Accord Sport I got the splash guards and trunk tray from handa-accessories.com. One thing I noticed is that it was actually cheaper to place two separate orders because when I combined everything the shipping went WAY up.