You can use whatever version of citrate you have handy, including citric acid. It is just a matter of adjusting the pH correctly. Remember the three pKa values - if you start with Na3-Citare, you will use HCl to adjust the pH and end up with some NaCl. Based on 3 pKa values of COOH groups, you could minimize NaCl amount if you begin with mono-Na salt.
You could also use Glycine buffer or you can simply use 1x10-4N HCl, as I do.