I'm having a hard time focusing right now, let alone entering anything approaching a "flow state". But one thing that does get me some of the way there is ambient sounds; the noise of a cafe especially not only evokes happier times but is also proven (citation needed) to induce creative flow (it's something to do with allowing your brain to be a tiny bit distracted by background noise to distract the "inner critic"). Anyway, here are some of the best sources of ambient sound that I've found:

The Coffitivity site is basic but delivers very well on the sounds of morning coffee and of university students talking and working quietly.

Highlighted on MessyNessyChic, an hour of ambience from a Paris Bistro (warning, may be interrupted by irritating YouTube commercials):

The Ambient Mixer has a bunch of preset soundscapes that are all adjustable, and it includes the ability to create your own. This is great, but might be a procrastination opportunity too far.

For those who want to really dig into the noir future we seem to find ourselves living in, there's 12 hours of ambient sound from Deckard's apartment in Blade Runner. Despite this being fully dystopian, I have actually found this alarmingly easy to work to:

Finally, I've mentioned this here before, but YouAreListeningToLosAngeles combines ambient music with LAPD radio traffic to quite an extraordinary effect. The site allows you to choose other radio traffic from other cities, but I've always found the original LA one to be the most evocative. Very good for writing crime stories.

Hope this helps.