I second Eaton. I haven't purchased springs from them, but I have purchased the spring "liner" that keeps the leaf springs apart. They didn't have it in stock but ordered it in and sold me what I needed.
In addition, they discussed with me the problems with my plan of powder coating springs - the temp goes too high for the temper of the spring. So they recommended painting them, which I'm doing.
Gary, AKA "Gary fellow":
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Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI