Probably acrylic. The other choice is metal. Don't pay a lot for acrylic -- it doesn't wear well, even when it's clear. Actually, I can't think of anything that would have a real mirror effect except the real thing.

'Pup

You can use silver leaf

Do regular mirror's weather outside okay? Even if exposed to cold?

Lisa

I've been hunting for an online source for acrylic mirrors. It should come in rolls and is frequently used in items for kids rooms. I thing we actually see much more it than we realize. Just check out the inexpensive mirrors at places like Wal-Mart. They may all be plastic, rather than glass.

Have you checked your local big box like Lowes?

Gloria

Any plastic shop will carry sheets of reflective-coated acrylic or plexiglas coated that look just like mirrors. (I bought some when my daughter was a baby so she could have a non-breakable mirror attached to the inside of her crib.) They usually put a tough, second coating on the back to protect the reflective coating. And, you can cut to size on a table saw, if needed. I think these would hold up fine.

Home Depot sells the acrylic mirror.

I have some misgivings about using mirrors outdoors. Although it can look very nice, some birds will batter themselves to death against one, either flying into it because they don't realize it's a barrier, or attacking the "other bird" that is intruding on their territory. You might want to think about that.

Home Depot and Lowes sells the outdoor acrylic mirror. Is #1 choice for standing up to the cold winter months.
Checkout the mirror ponds I've made.
Today I'm going back for more. Extending it on a larger scale.

Here is a link that might be useful: Mirror pond/garden Click to see.