Yes springs are a fatiguing part, and will lose their spring rate over time. You should also strongly consider replacing the shocks while the rear suspension is apart.
I'm guessing this is a Camaro? As far as multileaf springs go, I think the primary outfits are Global West, Hotchkis, and Detroit Speed & Engineering..I'm sure there are others. And there are quite a few dealers for all of these. I converted my car over from mono to multi using Global West leafs and the Detroit Speed conversion kit. You can either reuse your monoleaf shock plates and just get new shocks, or you can go with a multileaf shock plate and multileaf shocks.
My buildup in my signature for a bit more information.
