I purchased a Nikon SB-30 flash unit for the DP1. This is a small flash originally produced by Nikon for their film cameras. It's still available new from Nikon for under $90.00. I got mine through Amazon.com. Rated at guide number 52 I believe that this flash is more powerful than the Sigma small flash, the EF-140. It is designed to cover an area for a 28mm lens, which works out just fine. I think the SB-30 is about the same size as the EF-140 and it comes with a nylon case with a belt loop attachment.
The disadvantage to this unit is that it's not a ttl flash for the DP1. This is of almost no consequence for me. When I was able to purchase my first external flash, 40 years ago, there was no such thing as ttl. All flash photography involved math...rudimentary math. It's not really hard. In addition this flash unit has several settings for various f-stops and film speeds so a quasi-ttl could be set up if I wished. What I usually do is take a quick test shot, review it on the LCD screen and make adjustments if needed. Then I'm pretty much good to go.
I set the camera to M. As it's the f stop that determines image brightness I can set the shutter for whatever creative urges I have. I generally set the flash to 1/8 if within 7 feet of my subject and 1/1 if further away.
Interestingly this flash recycles so quickly that at 1/8 power with the DP1 on continuous drive mode the flash fires for all 3 shots. (at 1/1 I seem to get flash for exposures 1 and 3). I use this often if at the various blues joints I frequent. As this camera looks like any other point and shoot the 3 quick flashes are not any more annoying than the "red-eye" flash setting others are using.
Another feature is the "Wireless" flash setting. When set on "M" the flash will fire in sync with the pop up flash on the DP1. This works well for off camera flash for up to about 25'. The "A" setting is for sync at a greater distance but the DP1 doesn't seem to trigger it...the Nikon flashes do though.
This flash also has adjustment capabilities (+1/2, -1/2) as well as a diffusion screen for close-up.