Why go through the expense of a commissionable broker? You'd be better off going station-to-station for a "time buy" or even an LMA with the ownership of what station(s) you want to buy time on.
Few "brokers" who buy and sell stations do time lease agreements outside of a full-LMA. There are brokers for specific programming buys, however, and they can be found online. Unless you're looking to lease time on a lot of stations, you'd be better to negotiate your own deal one-to-one with station ownership and management ... saving yourself a lot more money.