Look for glass beads made for reflectivity of highway striping paints.

They are probably much smaller than 5 to 10 mmm.

You might look at styrofoam beads, or pummice, ( not hollow, but light weight )

Thanks for your reply Michael. Basically i was looking for something that has high strength. 3M makes high strength low density glass spheres but they are less than a micron in size. Then there is a company in US that makes high strength light weight ceramic spheres, with size in older of few mm, but that is very expensive. Though styrofoam, pummice have low density but strength factor is lacking in them.

You might also look at expanded shale, it is cooked like popcorn so it pops and fuses.

the surface is highly porous, but it has some strength.