Airstreams only used leaf springs until '61 or so, all the trailers after that use a torsion axle.
New axles are available from a couple of different companies, the easiest way to get the right one is to go with a company like Colin Hyde or Inland RV who will know from the description and VIN of your trailer which axle to order, or with a little research you could order axles directly from a company like Redneck Trailer Supply. Doing some research will save you headaches and money later, there are a few options like increasing the load carrying rating of the axles, changing the start angle, etc. There's a ton of info on here about it.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill