Richard Sarles may be in charge of WMATA for the interim, but the transit agency still needs to find a permanent replacement for John Catoe. On Thursday, Metro’s board gave approval to hiring executive search firm Heidrick & Struggles to help them with the task. It’s going to cost the cash-strapped agency $150,000 to hire the firm for a roughly six-month search. Heidrick & Struggles has previously conducted searches for chief executives to lead the…