oh i LOVE mountain rose herbs! great tea selection, and bulk loose leaf, black, herbal, flowering tea, red tea, much of it organic. i buy a pound of gunpowder green and it lasts me a good 9 months-year and i use it a lot in cooking and for gifts.
Amazon frequently has some great deals on tea - I just bought organic chai rooibos (Yogi brand) for fairly cheap. I've also bought organic mother's milk lactation tea for WAY cheaper than I've ever seen it in stores. If you do the Subscribe and Save option you'll save an additional 15% and your tea will ship free - you can cancel right after making your order and you'll get the first order but no additional ones. I do it all the time - love it!
I buy looseleaf locally. some are organic, not all, and both are online.
http://shop.blueturtleteaandspice.com/ (This one you can actually sort by organic and fairtrade)
I personally have never purchased online from either, but speaking for them in the store, they're awesome.
I would recommend Numi
They tend to have interesting flavors, flowering teas, loose leaf or bagged. Chocolate peurh (no sugar + cocoa powder and nibs) is a standout.
Another option is coffee bean direct:
Not everything is organic, but some is, and pricing seems to be good.
For general teas, I go with Upton Tea Imports.
For Chinese teas, especially oolongs, I go with Life in Teacup.
They are both much cheaper and often higher quality than popular stores like Teavana.
Oh, and for finding new teas in general, I find Steepster to be excellent, as Melissa mentioned.
I really like the Tea Spot because of their ethical and environmental stance. Plus, they have great teas!
An organic tea sampler from the Tea Spot: http://theteaspot.com/organic-tea-sampler.html?catid=255
Amazon does have great deals. Some of our faves: http://www.amazon.com/gp/product/B001EPPF14
Not organic, but excellent quality: http://www.amazon.com/gp/product/B000ID34WU
On the cheaper end, ebay, I have found organic teas that are directly shipped from China, avoiding the middle man. For example: http://shop.ebay.com/i.html?_nkw=organic+pu%27erh&_sacat=0&_odkw=organic+pu%27uh&_osacat=0&_trksid=p3286.c0.m270.l1313
By the way, I just got an email from Tea Spot. They are having a 25% off sale on Organic teas today only for EarthDay. Promo code: earthday :)