Komplett do a fair few of these, for reasonable prices: [broken link removed]
I've never bought from them, but they seem to have a pretty good reputation by and large.
I recently bought (in Germany) an LG DVD burner, which
Writes to DVD-RAM (5x), DVD-RW (4x),DVD-R (16x), DVD+RW(8x),DVD+R(16x),CD-RW(32x) and CD-R(48x) media
connects to the laptop via USB interface. Nice drive and it was a good price in Germany - EUR100
Try eBay - usually lots of bargains there if you ensure that the seller is reputable. Alternatively try some of the sites mentioned on [broken link removed].
I bought one (internal dvd burner) from Komplett.ie about 6 months ago and its been working fine ever since. When I opened the parcel I was a bit bemused to find just a DVD Burner wrapped in bubble-wrap - I was expecting the box, cd with drivers, instruction manual etc but apparently what I purchased was just a physical burner. Installed no problem though, worked first time.....
i got 2 DVD burners from Komplett - one external and one internal - both worked first time and have continued to work hassle free. both were samsung. i can't remember exactly the prices but the external was about 70ish and the internal about 50e. i also got a firewire card from them as well and this also was and still is hassle free. a very good service from them also i think.