For me, it depends on the blade. For example, Astra SPs and Perma-Sharp Supers - Amazon Polsilver Super Iridiums - www.polsilver-blades.com Feathers - www.doubleedgeshavingplace.com
ebay for me. I won an auction for 100 Rapira Super Stainless for $5.79. They also have lots of 100 packs for under $10 with free shipping. http://www.ebay.com/itm/100-pc-GILL...495654?hash=item1c4b6ca526:g:sOkAAOSwiCRUl4pn
Check THIS THREAD to see what folks liked enough to make a 100 blade purchase. Might be some hints where to get them although it's a 7 month old thread.
Here is another source with free shipping and good prices: http://www.razorsdirect.com/page/page/637542.htm Click on Double Edge
All of my Russians are from Ebay. I have a group of Russian sellers I tend to buy from. Never had any issues and the extra few days to get them is made up by the cheaper price. I can buy 100 Voskhod for about ten bucks shipped from Russia. My boss, who ordered 100 Voskhod on my recommendation, chose Amazon to get his free two day shipping, and paid $18. Sure he got them in two days, but at almost twice the price.
Gotta agree with most on here, getting blades from Russia, with different brands available for around $10.00 for 100 blades shipped is a great deal. Also if you'd prefer Personna Lab Blues, you can get them for around $11.00 for 100 blades, give or take, and shipped from the US.
Try A Blade is great if you are still researching what blade works for you and your razor. I just ordered another ten different blades from them. 2 blades each of the ten brands to try out. I got an email almost immediately thanking me for the order and 2 days later they arrived in the mail.
Jason is a great guy. He is really a part of our little community, and provides a cool service without gouging.