It would be great if there was a REST API equivalent for the SOAP getLead call. getLead has the distinct advantage of being able to pass it a wide variety of inputs from Cookie value, SFDC ID, etc. There isn't anything on the REST side that is as flexible.
Get Leads By Filter Type(http://developers.marketo.com/documentation/rest/get-multiple-leads-by-filter-type/) should be capable of taking all of these as inputs. Is there something specific which you're unable to do with it?
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