Tanks are very simple to use and many work well on the iStick 30W. With the lip on the device, any tank that is 22mm or less in diameter will look nice, and any tank that has coils at .4ohms or higher will work.
You really have a ton of options that will be hard to fit all in here, but I can certainly give you some. A good, very inexpensive option is the eLeaf GS Air M or MS (mega or mega shorty). This is a non-subohm tank with 1.5 or 1.2 ohm coils; the 1.2 are the only ones with organic cotton so I'd recommend those. The tank itself only costs about $10 and the coils are inexpensive. They can be used anywhere from 10-25ish watts. ELeaf claims that they can be pushed up to 30W, but I think this is high.
An Aspire Nautilus Mini would also be a good choice, but is more expensive.
The iStick 30W is capable of subohming down to .4 ohms, which opens up your options quite a bit. I use a Kanger Subtank Mini and Nano on my own 30W, and both work well. Some other popular options are the Aspire Atlantis 2, Aspire's Triton (might be a bit big for the iStick), or the Eleaf Melo.
Personally, I would probably recommend the Eleaf GS Air M, just to get your feet wet if you've never used a tank. It's a pretty decent device and won't set you back too much. However, if price isn't really an issue then I might suggest going straight to a Kanger Subtank mini or Nano. These have stock coils available in 1.5, 1.2, and .5 ohm (and .15 ohm nickel coils that CANNOT be used on an iStick 30W), so you don't have to go straight to subohming. In fact I really like the 1.2 ohm coils on my 30W, but the .5s and 1.5s also work well.
Using a tank is exceedingly simple. When filling a bottom-fill tank, you unscrew the base. If it's time to put a new coil in, you'll unscrew the old one and screw in the new. Most tanks come with a (dry) coil already in there, so you'll see exactly where it goes. It's a good idea to put a couple of drops of liquid in the hole in the top of a new coil, as well as one or two drops in any wicking holes on the side. Then all you do is turn the tank upside down (with the base still off), and fill it up with
juice. Just make sure you fill down the side of the glass to avoid getting any juice in the "chimney," the tube that leads from the coil to the mouthpiece. Likewise, do not fill the tank higher than the chimney. Once filled, screw the base and coil onto the bottom of the tank, let it sit for a few minutes to saturate, and you're set.
If any of that is confusing, watching ANY youtube video about a tank will make it clear. I can't stress how simple it is. Tanks a great option for those who want a good vape with little fuss.
Good luck! Please don't hesitate to contact me directly if you have any more questions or if I've said anything that wasn't clear.