I haven't seen any commercially available purified dCas9 out there. I think your best bet is to either use a plasmid, as Simon suggested, or clone the gene under a T7 and make mRNA of dCas9.
If you're looking for recombinant protein - a number of companies seem to sell it (see below for examples). A simple Google search found these in less than 30 seconds...
If you are looking for plasmid expressing dCas9 then follow Simon's link - Addgene is the goto place for all sorts of plasmids nowadays and really can't beat the price!