This concrete block is a particular type called an arched, scalloped, concrete edge (or edger or edging), for walkways or garden edges.

One example:
http://www.countrystone.com/products/concrete/edgers/curved-scallop

I looked for the WHITE (there is also red), and without paint applied. (I think the red coloring and of course paint, would scrape off into tiny rattie nails and perhaps be digested.)

Mine is a broken part of a larger unit. You could call gardening centers in your area and ask them if they carry this type. Many gardening centers have broken bits of concrete in a pile, which are super-cheap or even free.

Even though I like my concrete block, I find that eventually I need my rats to get a more formal trim, and for that I take them to the vet, where one vet tech holds the rat, and the other clips the nails. I personally can't see that close up to do the clipping myself.