You could try either ticketmaster or ebay, i would imagine some Toulouse fans may now be trying to sell their tickets, as well as other clubs who have just been knocked out
The only tickets left are the ones reserved for each of the finalists and they will be given to the clubs concered to sell after the Semis.
Apart from that, the only places you'll find them now (other than corporate packages) is via resellers/touts such as ebay & Viagogo.
I would suggest waiting to see who the finalists are & then looking to see if fans of the losing Semi finalists want to sell any on that they may already have purchased.