So, can I get this at the dive shop? Not at Walmart?

Just be careful to get a
glass lensed magnifier. I get mine at hobby shows, flea markets, gun shows, etc. Pay a dollar or two. The higher the magnigfication the better- if it is almost unusable "in air" because of it's high power~ that's the one to buy.
You've got the right idea about picking a 3x3' spot and just staring at it. At CCV you can do this pretty easily on a shallow coral head on the shore dive in The Front Yard. If you
very slowly move along the Prince Albert shipwreck's side- about a foot away- concentrate with the deck itself
just at your eye level. Look for the critters that inhabit the rusted crack between the deck and the hull sides. This is a great perspective to get a side view of critters hiding in plain sight- on the deck itself.
One trick I have used to teach this art: Give the divers a tupperware box and lid. I punch a lot of holes in them with a hot nail. Tell them that you want them to collect any shell smaller than a nickel. When they come back, before placing the treasures back in the sea, they will be interested in what they found- and what starts to crawl away!
The real lesson that you have taught them? Adjust your eyes for "the small stuff". Forevermore you will have created a new macro diver. Downside? Sometimes you "can't see" the big stuff~ your eye's focus has been so retrained.
This is the case so much so that when we find something to look at, the first thing we signal is the creature's general size. I have stared for several minutes at a Scorpionfish and not seen it as I was looking for something macro-minute (not something that I had seen a thousand times). I never saw the Scorpionfish until either someone outlined it or it spooked. There I was, looking for a Pipe Fish or Decorator Crab.
I was so intrigued in a microscopic Crab that I darned near flippered a 9' Nurse Shark. :doh:
It's all in the small stuff. You'll have found a new home on the South side of Roatan. i hope you are grinning from ear to ear, and post a report on SB.
Here's what they saw at CoCoView last week:
Dockside Dive Center Log