If you're looking for Authentic designer bags you can buy directly online ie. ( Coach.com ) (louisvuitton.com) etc. Those are the only place I'd purchase from online. The actual Authentic websites.
The individual designer stores themselves. Coach. Louis Vuitton that you can look up. ( Google for your area or look on their website for store locations.) Also, Large department stores sell them. Macy's. Dillard's. Nordstroms. etc. and last but not least. If you have outlet Malls in your area. They're definitely worth going to. You can save quite a bit of money there.
If you're looking for designer inspired. They're pretty much available everywhere. Be careful though. You get what you pay for.
Hope this helps