I purchased two small glass dishes - 1 from GW and another from a glass store at an outlet center in PA.
The trick to this is doing many light coats. I also taped off the outside of the glass (you want to spray the mirror finish on the opposite side that you want the mirrored-look). After the paint dried-I painted over it with Martha Stewart craft paint in Black. Looks pretty good if I do say so myself...
I also used the same spray paint on an old piece of glass from this great vintage frame we found on Etsy. I scratched the back to get an antiqued-mirror effect.
The finial is another project I worked on a few days ago-I found it at GW for $2...you can see it's original finish below...definitely not working in my house in that state. I added some acrylic paint and I think it looks pretty great on my shelves next to my DIY mercury glass pieces.
Happy treasure hunting!