Etrailer.com, SDTruckSprings.com, SummitRacing.com. Ebay and a few more. See who has a sale right now. By the way, AirLift and PacBrake bags will do the same thing. I went with the Firestone as they were the cheapest at the time. Haven't regretted my airbag purchase.
2012 Sandpiper 365SAQ weighing @ 15k, Onan, 2nd air, slide toppers, TST
2013 F350 6.7L Lariat 4x4 CC DRW,Viair 1007, Ride-Rites, Edge CTS, B&W
2001 Chevy 3500CC DRW 4x4, 8.1L, 4.10 gears w/ Detroit TrueTrac
2001 Ford Excursion Limited 4x4 7.3L,V/B Spring Mod, ProComps, Hellwig Swaybar, & other 'Necessities'