Part number is W01-358-5412. It is made by Firestone. You can purchase at www.truckspring.com
for $125.39 each. They where in stock. This appears to be the correct bag for most of the Classics.
Parts for the Henschen tag axle can be obtained from American Carrier Systems 800-344-2174. The descriptions and part numbers are listed in the Tag axle drawings provided in another thread. Prices are very reasonable for example transverse rod bushings 7572-001 are $12. By the way if you are going to change the air springs you should also change the rod bushings (4)and the straddle pins (2) since these get considerable wear.