Check out the Ikelite AF-35 stobe. I have no experience with this particular strobe, but I am fond of Ikelite strobes and this one looks like it might be really nice on a P&S. It looks like the kit comes with a strobe, slave sensor, and arm for $425.
If you think that you might be upgrading camera systems sometime soon you might want to go for the DS-51 for a little more cash. Both strobes as similar but the DS-51 has a little more power.
Both strobes have a 70 degree coverage angle and 80 degrees with the diffuser, which should work with the wider 28 mm focal length of the 870.
Inon also makes some compact strobes that might work well for you.
Good luck,
Ryan