LED Group Buy and RapidLED are your friends - they'll have literally everything you need for it.
In terms of parts needed (not counting tools), you'll need:
LEDs (of course!)
1 or 2 heat sinks
Cooling fans
Bulk wire
Water-resistant wire nuts or quick-disconnect plugs
Drivers
Power supply/power cord for the driver (may or may not be necessary, depending on your drivers)
Dimming solution (controller or potentiometers, depending on how you want to control them)
Mounting solution
Thermal grease or thermal adhesive
Screws (if using a screw-on solution to affix the LEDs to the heatsink
There are quite a few driver options available these days, some of which (in my opinion) are much nicer than the old standby ELN-60-48D. There's nothing wrong with those drivers, of course - there's a reason that they've been used in DIY LED systems for years. The new Meanwell LDD drivers, though, have quite a bit in their favor, the main downside being that they need a separate power supply. They're inexpensive, though, which means you don't have to try to minimize the number of drivers that you use. The power supply does add to the price, of course, so overall you end up at about the same price as using your original driver, but with greater individual control of your LED channels.
Some other things to think about:
Where are the drivers going to live? That will help dictate how much wire you'll need.
How many drivers will you need? This is actually pretty simple to figure out. Each driver will have a maximum and a minimum voltage rating. The individual LEDs each have a stat called Forward Voltage. To figure out how many drivers you need, just add up the forward voltage on each LED in a string. As long as you're within the voltage range for the driver, you can fit that string onto one driver. If you're below the voltage rating, you'll either need to add LEDs to get above that level or move those LEDs to a string with some spare voltage. If you're above the voltage, either add another driver or reduce the number of LEDs on that string. Having too few or too many LEDs on a string can damage them, so this is definitely something where you want to double-check your work before you finalize your design.
How are the LEDs going to be affixed to the heat sink? That could affect your choice of heat sink, and will determine whether you need to buy thermal grease or thermal adhesive.
How are you going to mount the fan onto the heatsink? A fan is not optional; LEDs put out a lot of heat, and if you don't have an active cooling solution, they will fail early. And by fail, I mean melt.
How are the LEDs going to be arranged? Your arrangement can help eliminate the "disco" effect, and it can also influence your PAR.
Are you going to solder or use solderless LEDs? Soldering is cheaper and more flexible, but more time-consuming and you have to actually solder. Solderless are usually $0.25-$0.50 more per LED, but they're much faster to assemble. On the other hand, though, solderless LEDs are bulkier, which can restrict your ability to cluster the LEDs.
Can you physically wire up the LEDs with your preferred arrangement? Don't neglect the physical realities of wires and LEDs. Wires, especially solid core wires (you should be using braided wire, but just in case you have solid core) can only bend so much, and insulation can melt while you solder, especially if the wire is really short. Make sure that you can get your wires where they need to go without blocking the actual lamp.
Do you have a splashguard to protect your fixture? Even if your tank has a top, you still need a splashguard. Even a little bit of salt spray on an LED can lead to reduced performance or early failure.
Will you be using lenses? Not every situation needs lenses; if your tank is shallow or you have an overpowering number of strong LEDs, you might have enough penetration without lenses.
Do you have enough clearance on your heatsink to mount the lenses underneath the splashguard? Most of the nicer heatsinks will include a channel to slide a splashguard through. Make sure that there's enough space to fit a lens underneath it.
How will you be dimming your LEDs? Technically you don't have to dim, but the benefits of dimming are so great that I don't consider that an optional feature. There are plenty of dimming solutions available, from simple potentiometer controls to elaborate controllers with sunrise/sunset controls and weather emulation settings. Any of them can work, just a matter of what you want to do.
Where will the dimming controls be located? They don't necessarily have to be easy to get to - once you get things dialed in, you shouldn't be fiddling with your dimming settings frequently. But it also doesn't hurt to make it easy to reach.
Building your own LED is definitely a lot of work, and compared to the early days, I'm honestly not sure how much money you actually save by going this route. But I've personally found it very rewarding - the ability to make it exactly the way you want it is, in my opinion, the best reward for building your own.
