I have a wooden slotted ruler rack, but I don't remember where I bought it. It was several years ago. I know it was mail order and it couldn't have been too expensive or I wouldn't have bought it. I just saw it in the most recent Clotilde catalog for about $15 and I'm sure it's available on their website
http://www.clotilde.com
It is 4" x 20" x 5/8" high and has 5 slots. It hold all 7 of my rulers and I consider it well worth what I paid. Maybe you can find it at your local fabric store.