I can't remember where I got it. I probably just googled for whitenoise, then looped a resulting mp3. I've tried some of the generator apps - some aren't bad - but I just have another app in the background running which I don't want all the time. A single looped mp3 I can play on any device. I've also got a sound machine I use for sleep - it's got a 'waterfall' sound I use as whitenoise to go to sleep.

http://indieconf.com/whitenoise15.mp3 is what I use most days.