I don't know about the 7800 in specific, but www.atlex.com
is a reputable company with great prices on Epson inks.
The Pro Epson 7800 & 9800 printers use pressurized ink cartridges. These are NOT the same as the 220ml cartridges for the 4800, which are non-pressurized. Currently (8/17/06) Atlex does not carry the pressurized versions of X800 cartridges. They may however be obtained from DTGweb at $85.00 ea. They also supply the Phatte Black cartridge for those who print with Image Print 6.1 for $ 100.00