It's been awhile since I've been in the parks, and they've been known to move merchandise around a bit, but last time I checked you could find D&B bags at:
Vault 28, Downtown Disney
WOD, Downtown Disney
Disney Clothiers, Main Street DL
Jewel of Orleans, New Orleans Square DL
Greetings From CA, Entry Plaza DCA
Gone Hollywood, Hollywood Backlot DCA
Grand Californian Gift Shop
Disneyland Hotel Gift Shop
Note: I haven't been in the Paradise Pier Gift Shop for ages, so no idea if they carry D&B bags or not.
That said, some stores carry limited styles. For example last time I checked DL Hotel Gift Shop only had the colored sketch collection, Jewel of Orleans only had the DL55 collection, etc.
In my experience it's definitely easiest to shop pattern placement at Vault 28 since they have a large display of bags and generally have the widest selection of bags in stock. Most of the other shops only have one or two of each style bag they carry on display (as MaryJ mentioned in her post above) and you may or may not be able to find a CM who is willing to bring more than one out from the back so you can compare placement.
Hope that helps.