Check your phone book. These are the "rock yard" style places that
have big ten foot wide bins of stuff. Usually contractors go there,
but all these places are open to the public.

One other thing:

Plastic. Cement. Is. Not. Concrete.

It cures differently, doesn't crack nearly so easily, can be manipulated
casually, and so forth. It's often sold as a sealant (e.g., Thoroseal).
And generally can be found in very low PH formulas.

C//