A holster-style bag like the Think Tank Digital Holster 10 V2 may be too tight if you want to carry the camera with the 18-200 mounted plus the SB-400 inside the main compartment. It might work only if the flash fits in an external attachment pocket.

For your kit, a small shoulder or messenger-style bag is a better match. Community recommendations included the Crumpler 5 Million Dollar Home and the Lowepro Exchange Messenger. Both are described as compact, over-the-shoulder options that can handle a minimal walk-around setup, and they’re also less conspicuous than a dedicated holster.

The safest advice is to try the bag in person if possible. Bag dimensions can be misleading online, and fit depends on how you want to pack the camera, lens, and flash. In general, choose a slightly wider shoulder bag rather than a tight holster if you want easy access and guaranteed room for all three items.