The biggest reason you can't find a air meter is because, compared to gas pumps, they didn't make that many of them. The trusty old air meter hung on the side of the station, long after the gas pumps were changed out three or four times. Luckily, you can still restore a ECO Model 90 Series meter because some of us do reproduce some of the parts and in my case I purchased over $13,000 worth of parts when the company closed in 2013.
One other thing I have been watching is the price people are getting for the unrestored ECO Model 90 meters, what used to sell for $300-$400 at the beginning of 2018 are now selling for $900 or more. And they are selling. The price of so called "restored" units is approaching $4000 and some of the sellers are stating "I don't know if it works or not, I don't have anything to test it." Now, who in this hobby doesn't own a car with tires and a air compressor that you can buy for less than $50.00 that will operate the ECO? Someone once said "You get what you paid for." No, many time you don't get what you paid for, you get less.
One piece of advice for anyone who is looking to purchase any air meter, especially on Ebay. Some of these sellers are just showing a few pictures of the meters, if you are going to pay $800 to $1000 for a meter you need a lot of pictures. Does the housing have rust holes or dents, are all the interior parts all there and what condition are they in, and what else is missing? If the seller isn't showing a picture of the interior parts, ask him to post more pictures. If he doesn't post them assume he is trying to hide something.
Author, 1st & 2nd editions of Gas Pump ID book, 3rd edition is now available at www.gaspumpbible.com
Air Meter ID book also available