There are several places that offer spacers. There are a couple of companies that make them that are advertised in the back of Diesel Power magazne. Also found these ones on ebay.
eBay Motors: BILLET ALUMINUM WHEEL SPACER 8 Lug Superduty 1/2" (item 110358748243 end time Mar-09-09 12:54:21 PDT)
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2007 3500 QCLB, 5.9, ISSPRO, AFE Stage 2, AF1 intake horn, Smarty, SB 3600#, Straight Piped with 6" Mitres, 6" lift, Bilstein 5100s/dual steering stabs., trac bars 37" BFG M/Ts,
CUMMINS FORUM STICKER!May 2009 ROTM-Thanks to James @ RSD for stepping up!
2006 3500 QC Bighorn, 6 speed, No silencer ring, B&W turnover, other than that, stock
"I Caught that diesel thing, now it stings when I pee"