If you're in the U.S., check out Dell Outlet. They have a bunch of base models listed for $179 and $189 today (20+). They usually go quickly - I don't think I've ever seen this many at once, or for an entire day (so far) like I have today. The cheapest I've ever seen has been $169, but I think that was for a scratch & dent model. The best part about the Outlet is that they usually ship within a day or two because the unit is pre-built, plus they offer free shipping if you choose the 3-5 day option.
If you want to order a new one directly from Dell, there have been a number of $199 sales for the base model. I think they've been happening every 3-4 weeks or so. The last one was around March 24, so there might be another one in a couple of weeks.
I've only purchased Minis directly from Dell (either through the Outlet or using my EPP discount on new systems) so I can't recommend anywhere else. However, I'm sure others will have some advice on where to check.
Mini 9 (910): Cherry Red, N270, 2 GB RAM, 16 GB STEC SSD, XP Home
Mini 10v (1011): Passion Purple, N270, 1 GB RAM, 160 GB HDD, XP Home
Inspiron 11z (1110): Passion Purple, SU4100, 2 GB RAM, 250 GB HDD, Win7
Alienware M11x R1: Cosmic Black, SU7300, 4 GB RAM, 160 GB HDD, Win7
Latitude 2110: Blue Ribbon, N470, 2 GB RAM, 250 GB HDD, Touchscreen, Win7
Zino HD (400): True Blue, Athlon X2 3250e, 4 GB RAM, 500 GB HDD, Win7
Plus a few others not discussed here...