Fortunately, you have many options.
First of all - what type of fountain pen is it? Is it a luxury pen or a vintage pen (the ones that cost over 100$)? Or is it an entry level pen in the 10-50$ price range? This makes a difference, because you certainly wouldn´t want to "experiment" with cheap alternative inks in an expensive pen and risk damaging it. Generally, if you´re careful, you won´t damage the pen, but you have to assess your risks.
Save your empty cartridges, as they can be refilled using bottled ink and a syringe (just be sure that the syringe is blunted so you don´t prick yourself with it). The syringes that come with printer ink refill kits are perfect for this task.
Since you didn´t say what type of pen you have, I´ll give you the pros vs cons run down.
Option #1 Bottled fountain pen ink
PROS
*specifically designed for fountain pens and therefore least likely to damage your pen. Some of the older brands of inks (like Parker or Waterman) have been in use for many decades and are therefore tried and true
*most are watersoluble inks that can easily be cleaned from the pen with plain water
*richest colored ink
CONS
*most (with the exception of some like Noodler´s Bulletproof Inks) are watersoluble and therefore are not permanent and will bleed/fade if they get wet
*expensive: generally you should expect to pay about 5$ an ounce for this type of ink. That may not sound like a lot of ink, but keep in mind that those cartridges you´re using are generally 1/2ml to 1ml of ink, so one 30ml bottle of ink will be worth 30-60 cartridges.
Option #2: Blick Black Cat India Ink
(I´m listing Black Cat India Ink specifically because it is the only one I have tried so I know that it works)
PROS
*Very inexpensive ink: 1 pint costs 7.35$+shipping from www.dickblick.com. Comes out to about 1$ an ounce with shipping...even cheaper if you have a Dick Blick art store near you.
*Nice warm charcoal black color that holds its own next to some of the more expensive brands, though side by side, it is clearly not as dark as, say, Noodler´s black, which is jet black.
*Flows nicely. It is a very wet ink and therefore gets pretty good flow regardless of how fine your nib is.
*Completely waterproof when dry
CONS
*If it dries up in your pen, say goodbye to your pen unless you have an ultrasonic cleaner (they cost about 50$). An ultrasonic cleaner will clean out even the worst of dried ink clogs. Keep in mind though that some of pens may have plastics or adhesives, etc. that are not ultrasonic cleaner safe. Generally, the only time dried ink becomes a problem is if you leave your pen sitting unused for prolonged periods of time (like over a week). If you use India Ink, it is advisable that you clean your pen with every refill, and clean it at least once a week. Otherwise you may run into problems that could potentially damage your pen.
*It settles. India Ink is pigmented ink - that means that it has small microscopic particles which can settle. It is best to use your pen frequently to keep ink from settling inside your pen, and never leave it sitting unused with India Ink in it for more than a few days.
*Generally speaking, Black Cat India Ink is the most likely to damage your pen, but if you know what you´re doing, it is perfectly viable, and I have never had any problems nor lost any pens using it.
Option #3: bottled Ink Jet Printer ink refills
PROS
*Cheap: 15$ will get you about 600ml (20oz) of ink, making this probably the cheapest alternative.
*Designed for printerheads, and are thus specifically designed NOT to clog small capillary-thin channels.
*If you buy a refill kit that comes with all 4 printer colors (Cyan, Magenta, Yellow, and Black), the inks can be mixed to obtain almost any color ink you want.
*Watersoluble and therefore should probably be fairly fountain pen safe
CONS
*not really tried and true (Personally my experience using this type of ink in fountain pens is very limited. I have tried it and know that it works, but the long term effects, if any, it will have on the pen are unknown. For example, I have had neglected pens completely dry up with solidified Black Cat India Ink, which I´ve been using for years, and I restored those pens to working condition again by putting it through the ultrasonic cleaner several times. So I know this ink well and its limitations and its potential for pen damage. Printer Ink I´ve only experimented with recently and therefore cannot tell you how it will behave once dried in the pen or how it will affect the pen after years of use.)
*Stains. Though this ink remains water soluble on paper and smooth surfaces, if you get it on your hands, you will have a very hard time getting it out. It is best to wear plastic gloves while handling printer ink.