Beaches
Searching most beaches should result in a large amount of rocks that are perfect for polishing. As the tide rolls in, it brings with it numerous rocks that were once on the seabed floor. Many of these rocks will already have a smooth texture that are ideal for use in jewelry or polishing. It's usually best to concentrate on the edge of the shoreline for optimal results.
Rivers
Looking along the banks and gravel deposits of rivers is another way to unearth a multitude of rocks. Usually smaller rocks suitable for polishing will be scattered around larger rocks that bridge the sides of a river. Depending upon the location, you can often find rocks of varying colors and textures.
Lakes
Walking around the shores of a lake is another technique for finding quality rocks. Although there is likely to be less quantity as compared to beaches and rivers, there is always a decent chance of coming up with some unique finds. Use a large stick to dig around in the shallow sediment at the edge of a lake; this can unearth beautiful rocks that are otherwise hidden.
Hills or Mountains
An additional place in which you can often find rocks is on the tops of hills or the ravines of mountains. When rain washes away the soil, rocks are exposed. Rainwater will carry away lighter debris, leaving most rocks in place. These are often the types of rocks that will be suitable for using in a rock tumbler.
Road Cuts and Construction Sites
Check out any areas where the ground is being disturbed, such as construction sites for roads or highways. Pay attention to any road cuts; these are banks of rock that had to be blasted through to create a passage for the road. Many loose rocks can be found in these areas.