Don't know what kind of "ornaments" you want to mosaic, but you KNOW what I do. You can also use polystyrene. Check out Sherri Warner Hunter's book on building with it.

are you looking for concrete items?

I'd think you'd just need to search thrift stores and garage sales. I'd be careful with places like Wal-Mart, Lowes etc. Lots of these ornaments are made out of plastics and resins. Not sure how well they would hold up to the weather. You want to make sure your base is made out of concrete and is not coated with anything that's going to wear off, like paint.

Garden centers, nurseries usually have concrete statues. Do a Google search for concrete statuary in your area. Never have found a cheap hobby.

WATCH OUT!!! My life changed from thinking I needed to find garden statues to mosaic!!!! LOL! I ended up taking a course that changed my LIFE! HA!Look at Rianna!!!

Here is a link that might be useful: How to make garden statues!

sometimes you can find a concrete casting company and they'll sell wholesale to you if you buy enough all at once... But I'm more likely to follow Calam and Slow's examples and make my own stuff