Purchase a frame from Michaels or Joanns. The frame I used is priced at $40 at my local Michaels and was purchased for $12! To enjoy an awesome deal like that download yourself the AP for Michaels and Joanns on iPhone or droid and double up on your coupons, bonus if the item is already on sale!
The frame I used is 11×14 but any size will work. Take the guts out of your frame (the backing and glass.) Go to a local glass store to purchase the mirror. Craft stores do not carry the mirror. I went all over town and no luck; so to save yourself some time go straight to a local glass store. I purchased mine at Osceola Glass in Saint Cloud, FL. They were very polite and helpful and didn’t seem to mind I was making a small order. The mirror cost me $9. Make sure you take the frame with you to ensure the mirror is cut to the perfect size. I didn’t bring mine with me and the mirror was just a tad bigger then 11×14, so I just used my E6000 and I glued the mirror to the back of the frame. I then taped the cardboard from the guts of the frame to the back of the mirror so that it would not scratch on the wall and lose the reflection.
If your mirror fits snug in the frame just put the backing on from the guts of the frame and Viola! you have your self a DIY Framed Mirror that cost $21! A framed mirror like this could cost you up to $60 if purchased from a home store.