I've typically found them at about $50/cartridge if you shop a bit. I've found good pricing at Amazon and Staples (with 20% off $100 purchase coupons). I think I might have purchased my most recent cartridge from Red River Paper with a paper order.
The only consideration would be to purchase from someone who moves enough ink to not ship you old stock.
I usually order inks from Alex, but you are right, B&H is cheaper, mostly because they don't charge for shipping. For the 3880, B&H is $15 less. For my 4900, the cartridges cost the same $86.95, but Atlex charges $11.69 for shipping a cartridge, whereas B&H charges nothing. Of course, if you buy several cartridges, the relative difference becomes minor.
I checked Hot Press Natural 13x19 paper (25 sheets): Atlex charges $3 less than B&H but you have to pay for shipping; for B&H, no shipping charge.
In the future, I will always check out B&H. They are definitely competitive.
Normally I just order from Atlex and they are almost $2 cheaper per cartridge than B&H on ink for my R3000. With free B&H shipping it may be a wash. On other Epson printers B&H looks to be priced the same as Atlex with free shipping to boot. So it is clearly worthwhile to compare. You can't go wrong dealing with either IMO.
I usually buy ink for my 3880 from Adorama. They are competitive in price, and I can be confident that it is fresh. If you find ink from some obscure source for a lot less, it might be close to the expiration date.
Thanks everyone for all the info. So far B&H seems the most cost effective hands down. I am pretty sure their supply will be fresh. The ink I just got said use before 01/2015. Seems Vivid light Magenta went first followed by light black with photo black right behind. Vivid Magenta & Cyan have been hardly touched, strange...