For retail shops with significant O gauge postwar stock, the only local choice is Sommerfeld’s Trains in Butler. Jack also has a great selection of new modern O gauge locos.
Hiawatha Hobbies in Waukesha (Pewaukee postal address) occasionally has several pieces as well, but the focus there is new stuff and lots of N and HO, as well as other hobby lines.
Both are worth a visit.
By the way, I work at Hiawatha a couple of days per week, so maybe I’ll see you there.