Where can I buy tickets for the Chicago Cubs vs. the Pittsburgh
Pirates on 6/5 or 6/6 for face value?

It's going to be hard to find tickets at face value for any cubs games
this season.  Your best chance to score tickets is to continue to
check out cubs.com, because they continually release tickets, if
someone's credit card failed to go through or something similiar to
that.  Also if your willing to accept limited view seats, call the
wrigley field directly, 773-404-cubs, i believe.  Worse case scenario
is to get to the park early that day and you can get standing room
only tickets.  Good Luck