I would advise refilling with bulk ink, rather than buying compatible
cartridges. If you buy the inks in 500ml bottles (here in Japan you
can buy anything from small 50ml or 100ml bottles right up to quarter
gallon bottles!). The initial sets you need should include the tool
for opening up the hole in the cartridges. After that, just refill as
you need.
You will also need a chip resetter (stand-alone or USB-powered are common).
I did do that with my old Canon iP4300, but I found that refilling Canon
cartridges was a pretty messy operation! I then bought some third-party
refillable cartridges, with bottles of bulk ink, but these did not
indicate ink levels properly. Well they did, but if you turned the
printer off the ink levels automatically returned to "full" when you
switched it on again!
The blue section of the print-head also stopped working properly soon
after I started using these refill cartridges, and I rather suspect that
either the cartridges or the ink may have had something to do with it.
One thing that annoys me about the new iP4950 printer is that the
cartridges hold a lot less ink than those on the iP4300. Also when I
turn the printer on and do some printing of DVDs (the main thing I use
it for) the printer spends a long time clanking and chunking about
getting ready. This seems to use a lot of ink for nothing - after
printing about 40 disks since new the cartridges are already showing
half-empty, which seems excessive.