The lease market is through peakmonsters.com. That's where I do all the leasing, etc. It's a project by the team behind @steempeak, but I believe they collaborated with @steemmonsters for the project, too.

The website is actually a well-developed marketplace for Steem Monsters/Splinterlands cards. You can buy cards in bulk, bid on cards for sale, and lease. They also have a pretty good block explorer for all steem monsters activities. You should check it out if you decide to go back into the game, too.