We have a hotel liquidators in our area and they have the best prices on mirrors.

You could also check places like home goods.

You could always have a frame made too, but anything custom tends to be more expensive.

Homegoods for sure. I purchased this mirror there for $40 a couple of years ago. It's 29 x 35, nice and big. Will probably end up spray painting it (won't be using any gold tones in the new home) and using it in the powder room. They had a huge selection there as well, love that place!

This post was edited by MrsB1227 on Wed, Dec 24, 14 at 9:39

Our trim carpenter builds a custom frame and then mirror is cut to fit. Looks pretty nice.

You can find mirrors at antique malls. I found my powder room one there for $25. Overstock and home goods are good sources too.

This one is from overstock:

you can by frames that fir over plate mirrors.

Here is a link that might be useful: try this solution

Ikea has a limited selection, but if you need big ones, that is a good place.
Also, have you just tried a custom glass shop? I found that they were quite reasonable.

And, Craigslist isn't bad either.

Similar to HerrDoktorProfessor's suggestion,you can buy picture frames and have a glass place cut a mirror to fit. Also, you might be surprised - we had a large mirror framed by our cabinet guy and it was quite inexpensive.

Cabinet companies supply mirror trim as well. That's what we're going to do....that way it will match the cabinets.