The last couple of years for my 9900, I've gone to eBay. When I can't find what I need or need it quickly, I go to B&H, Inkjetart.com, Lexjet.com. All offer discounted pricing on both paper and ink.
I wouldn't even think of using 3rd party ink since you don't know what you'll get. The cost saving per ml isn't worth fooling with compared to simply using the OEM inks.