Here are the compatible ink refills we offer for the HP 40 Black (51640A) inkjet cartridge:
Uni-Kit Black Inkjet Refill Kit - 96ml black ink
$6.55
Black - DuraFIRM Bulk printer ink - 60ml - 2oz
$3.95
Injector Kit for Inkjet Cartridge Refills
$2.65
Inkjet Cartridge and Printhead Cleaner - 60ml - 2oz
$2.75