The latest thing in men's work casual at my husband's office is a solid color t-shirt (a tasteful design on the front is OK) with an oxford shirt over top (not a full dress shirt - those sort of Land's End-y in-between shirts) in a matching color. E.g., a dark blue t-shirt with a medium-blue button shirt over top.
By far our favorite place to shop (for him or me) is Turn Style. It's a consignment store with several locations (listed on their website: http://www.turnstyleconsign.com/). The one we go to is in Eden Prairie. They're very picky about what they have, and frequently have very high end labels (Christian Dior, Armani, etc.).
My husband was really skeptical of shopping "used" clothes at first, but enjoyed both finding nice stuff for a good price and having all the shirts he wants to look at on one or two racks. I found that the best trick is to bring them home and wash them (even though they're already clean). It's sort of like claiming them for the family.