I'm using a Lowepro S&F (Street and Field) Utility Case (actual name, no product number I could find). The 420EX lies flat in the bottom. I lock it in with the supplied velco partition turned on its side and put the camera in (back down with a Lensmate attached) on top. There's still some room at one end of the camera for something that's about 2x3x3 inches.
There's an outer pocket for spare batteries for both items, plus other odd bits like remote controller, pens and paper, mints and keys and what-have-you.
If you find the bag on the Lowepro site, don't worry about the specifications. It says it's about 1.5 inches thick. When mine is packed its more like five or six inches.
I paid less than US$30 for it, but I'm in Hong Kong so I don't know what it is in the States. Less, I would imagine. I added a Domke no-slip strap to complete the unit. It's small and light and comfortable and I take it with me almost everywhere.