I've recently come across this lovely shop that has some of the best well made modest and plain cape dresses and aprons I've ever seen. The dresses zip up in the back with zippers and the aprons tie and button in the back. I'd make sure she knew you wanted that in the notes as she also makes items for the Amish who use pins and safety pins.
She has different styles of sleeves, full with elastic, full straight, and a 3/4 sleeve. The skirts come a-line, gathered and pleated. She has a nice selection of plain cotton fabrics in nice colors and also can do custom designs and use ones own fabric if you want a different fabric than ones she has. Click on the link above and order your new plain and beautiful fashions.