Ignore all the idiots out there on the web. Arduinos have no problems at all driving WS2811 LED strips. I've even done it at 8MHz on an ATtiny85 (in fact I'm doing it right now for a new gadget I'm building).
Buying a pre-built strip is best because each LED needs a resistor and a capacitor. If you buy the individual LEDs it's a lot more work.
If you want a complete strip, decide how many LEDs you want per meter. If you're going to chop it up, look at the price-per LED.
Any of these will do: http://www.ebay.com/sch/i.html?_nkw=ws2811+led+strip