I'd sure try a NAPA >truck< parts house if you have one close by. I've had to replace some strange air springs several times before that other dealer related shops said were not able to get and my NAPA parts house has always been able to get them next day or two, if not already in stock. You have the part number, no need in telling them what it mounts to, as it may well confuse the parts clerk. especially if it is an idiot kid with little or no real world mechanical knowledge.
2006 Excel 36BDO
2012 Dodge 3500