Note: Since it's hard to buy scuba tanks and get them filled without a certification card, I'm assuming here that anyone interested in putting a scuba air tool rig together is familiar with the dive gear side. No links to diving equipment are provided except for the adapter to get into the air tool domain. If you're interested in this sort of thing but don't have a tank and access to air, consider POWERTANK, a liquid CO2 arrangement that gets good reviews and looks like it's more portable than a comparable scuba system. I've heard it's safer too, though the danger of transporting scuba tanks in cars is often overblown. Working with high pressure gas can be dangerous. You assume all responsibility for safety if referring to this page for information. Caveats for 21st century morons are omitted. If that's a problem, please read no further. Glossary at the bottom — here. Thanks to Andrew Sharp for his post at afmracing.org that gave me a great start.

I have not tested this system underwater, nor does this page address such use.

Why use a scuba tank for air tools?
Not much reason unless you have the scuba stuff already and would benefit from a portable, short-burst, limited-capacity air supply for one or more limited duration tools (not sanders, angle grinders, or similar tools when used continuously).

Below is my setup with an impact wrench (in attachment order from left to right), a perfect match for this type of air supply. Some pics are clickable for a bigger version. Links are for equipment info and pics only, since you can find lower prices elsewhere for most of them:

That setup's what I chose, but there are many ways to do it. Some prefer a welding regulator in place of the 2 regs in this scheme. Or, depending on the air tool and its air/lube requirements, it's plausible to use only the scuba reg (no downstream reg/filter/lube). Scuba air is notoriously clean. Ultimately depends on how hot your first stage scuba reg is though.

What I paid to add air tool ability

Pros and cons

Pros
• Portable.
• Fast recovery because of the high tank pressure.
• More than enough burst power for most air tools. My impact wrench (pictured above) needs between 5 and 24 CFM depending on load, and it works well here.
• Lasts longer than most people would figure.
• For many, finding a dive shop to refill the tank is easier than finding places to refill other kinds of tanks, like Nitrogen or CO2. (Non-divers may be able to arrange a "no C-card" situation with a fill station if the air's to be used only for tools. If so, don't ruin your good deal by telling the wrong people which shop it is.)
• Saves buying a compressor with comparable performance if only needed on occasion.

Cons
• The usual tank hassles (e.g., fill scarcity/expense; weight; rolling around during transport; possible missile/rupture hazard during a collision).
• Peak pressure dies off fairly quickly, probably because of constrictions between the tank valve and the downstream regulator. Scuba regs aren't designed for prolonged, high-volume operation, and the skinny BC hose doesn't help. If you go in knowing that, it can be worked around easily for most intermittent applications.
• Can be a drag to either haul the second stage regs around or disconnect them (assuming you dive with the same first stage).
• Slight performance change as tank empties. Can be tweaked with the second, adjustable regulator.

Buying parts
Home Depot has a pretty good selection, though I doubt you'll find the prefab Reg/Filter/Lubricator there. They should have enough separate components to put something together. I bought almost everything online, but at a place I wouldn't buy from again (nincompoops). CARQUEST has an amazing selection of air tool hardware, as do some ma & pa hardware stores.

The toughest part to find may be the BC to 1/4" NPT adapter. You can try here and here (part code 57300), but you might just want to drop by your local dive shop, buy one of their usual BC Tire Inflator deals like this, then remove the first section — which appears to be the exact adapter that's needed. There aren't many shops that sell it by itself, though they can probably order it (Trident part code AA04). If you google the part and find your way to "Mike's Scuba", let me recommend that you buy a metal lathe and build whatever you need before messing with them.

1/4” Automotive connector kit
There may be some things you won't use, but it's cheaper sometimes.

Air tool oil (Clean regular SAE30 motor oil works too.)

Teflon tape No leaks. Easy to use.