Fons and Porter's quilt shop is in Winterset. I like Creekside Quilting and the Quilt Block in the Des Moines area, The Quilters Cupboard in Ankeny, and the Quilting Connection in Ames. Depending on the route you take from Winterset to Wisconsin, you might like a stop in Kalona at Stich-n-Sew.
Then there's Fern Hill in South Amana and Heritage Designs in Main Amana, and in the Marion/Cedar Rapids area you'll find Pine Needles (Cedar Rapids) and Connie's Quilt Shop (Marion).
Hope that gives you some food for thought!