The David Rubenstein Atrium ticket outlet had discounted Mariinsky tickets for Tuesday night and today's matinee. There is no telling whether they will have any for future performances. However, a quick look at the Met and LCF websites shows that Thursday evening (A.K.) and the Saturday matinee (L.H.H.) are far from being sold out. So there's a reasonable chance that some of those tickets could be dumped at the Rubenstein.
Availability will be posted on the electronic board above the box office windows right around 11:30 am. Box office opens at noon.
Once in a while, the little poor people win.