Looks good!
The Critter carries the standard smaller black bulkheads in the front left of the store. Before you get there, you should know whether you need a threaded one, push fit, or one with a barb (which will all depend on your planned plumbing set-up). They usually have a couple of each, but depending on the size you might be out of luck.
FWIW, as you're planning how to plumb things, you should use a lot of unions and tru union ball valves. You should also plan to be able to take your bulkheads back out without having to cut them out (for me this usually means getting a push-fit on the flange side and threaded on the long end (so I can get the plumbing off of that end and pull the bulkhead through the tank). The reason I say to get these parts is that you'll inevitably want to change something about the set-up in the future or move the tank... and having to rebuy parts and/or replumb the tank is seriously time consuming and wasteful when a little forethought will get you something you can always work with easily even with the tank set-up and running.
Steinhouse Supply carries heavier duty gray bulkheads that are similar in size to the ones that glass cages tanks come with (except they are shaped like a giant nut, which is actually better for getting the things tightened in small spaces).
I would guess that if your tank was drilled at the factory, then the holes in the tank are too small to fit heavy duty ones. To my knowledge, if you can't find them at the critter and/or they can't order you one fast, you'll probably have to go online.
