You should be OK with Sainsmart, Geeetech or many of the other discount suppliers. There is at least one thread on the forums here about Sainsmart and as with most other companies there are good and bad experiences. Use the search here and you'll find many opinions.
The Arduino license encourages others to build compatable hardware. As long as it's not a counterfit part, meaning they are telling you it's a real Arduino and using the logo and trademark illegally, there isn't an issue with using a clone. Massimo Banzi (basically Mr Arduino) has stated he has no qualms about using clone products as long as they comply with the license. Open Hardware stalwarts like Adafruit and Sparkfun have both made variants of the boards at one time or another.
The Arduino license encourages others to build compatable hardware. As long as it's not a counterfit part, meaning they are telling you it's a real Arduino and using the logo and trademark illegally, there isn't an issue with using a clone. Massimo Banzi (basically Mr Arduino) has stated he has no qualms about using clone products as long as they comply with the license. Open Hardware stalwarts like Adafruit and Sparkfun have both made variants of the boards at one time or another.