When: My guess is the late 1950's. Anscochrome slides don't have the dating help that Kodachromes tend to have, but the Hertz truck in the frame with its dual headlights leaves me to think this was taken in the latter part of the Nifty Fifties!
Where: Frederick, MD. The slide mount certainly helped to identify it since it was labeled, but I could have determined this anyway, based in no small part to the prominence of the Francis Scott Key Hotel in the frame. Since this photo was taken, Frederick has undergone decades of change, and today, the Downtown area remains healthy, with a vibrant restaurant scene, boutique shopping, and a promenade built around Carroll Creek. The scene below was taken a week ago, but most of the holiday adornment now is in the form of lights in the trees, rather than the items hung about.
How (much): One of the best of a batch of slides I picked up on ebay for about $15. There were other good ones in the batch, but this one clearly took the cake for me.