In December, I was inspired by a post on the Makezine blog to search for retail LED Christmas lights, as a source of cheap LEDs. I couldn’t find any at Wal-Mart, but my wife found the Phillips LED Christmas lights at Target.
I wasn’t particularly impressed with the white ones–they looked very blue to me. But the blue ones were pretty, and bright!, so I bought a string. 60 LEDs for $12 is $.20 each, which is a pretty respectable price for blue LEDs (although you can do somewhat better on eBay buying from Hong Kong). And the review linked above doesn’t recommend these as a home lighting product (as packaged, they flicker just at the edge of perception), but they work great for cannibalizing. 
The LEDs have their leads stuck through little plastic bases, so they clip into and out of the plastic sockets much like other miniature Christmas lights. Happily, they also slip right out of the sockets, so they’re very easy to use.
I had an old stereo receiver with a burned out backlight on the mechanical tuner, and I’ve already soldered in a blue LED (with current-limiting resistor) to replace the original incandescent. It actually looks almost violet now, and I’ll probably replace it with a green LED the next time I have an excuse to open it–but it’s cool to have it working again.