The list of accessories for hydroponic growing systems is practically endless.  There are also countless manufacturers of these accessories.  It can be unbelievably mind numbing to figure out what you need and where to get it from.

My advice is to start as small as possible and go from there as you gain experience and confidence.  Ideally, you will start with an all-in-one hydroponic kit or system that includes everything you need to get going.  Circumstances, personal preferences and your own environment will have a lot to do with any additional equipment or accessories you might need.

One thing that's for sure is that the first "accessory" that you will need to contemplate buying is additional nutrients.  Nutrients are to hydroponics like razor blades are to Gillette.  They get used up and need to be replaced constantly.  Once you have your basic hydroponics setup, you should be able to get in several grows before you have to think of replacing any of the equipment, but it's guaranteed that you will need to replenish your nutrient supply.

Therefore, we recommend reading, experimenting and researching nutrients from day one.  Prices (including shipping rates) vary greatly.  I've tried Stealth Hydroponics Powder Nutrients, Advanced Nutrients as well as organic fertilizers and have not noticed any significant differences.  This is why I recommend the Stealth Hydroponics Powder Nutrients, they cost way less, their shipping costs way less, and they are extremely easy to use.

Your next accessory will probably be lighting.  Here again, you're going to want to get some research and experience under your belt before buying replacement bulbs.  I started with CFL (compact fluorescent lamps) because I had to worry about the electrical drain on my apartment's ancient wiring as well as the heat output of my lighting.  Your situation may vary. 

Additionally, water pumps, air pumps and air stones seem to have a limited life span and may last only a couple of growing cycles before needing replacement.  These items are all available at hardware and pet stores at reasonable prices so are no big deal.

If you grow from seeds, you'll eventually need replacement rock wool cubes.  If you buy clones from your pharmacy, they almost always come in rock wool cubes so you needn't be concerned with replacement rock wool cubes.

One final caution.  Hydroponic suppliers as a rule are not in a hurry.  So, if you're running low on nutrients or lamp bulbs you might want to order them as far in advance as possible.  Normal delivery times of three to four weeks seem to be the standard in the hydroponics industry.